We are given EXP boost represented by a + sign for Crime.Net jobs by playing various contracts. Reduction in EXP get as low as -30% and positive boosts get as high as +15%,and with stealth bonuses, you can reach an even higher boost, but more on that later.
Doing different jobs that have positive boosts or even negatives that are not at -30% already randomly spreads positive boost to randomly selected heist most of the time. If we keep playing the same contract over and over again we will see a reduction in the EXP up to -30%. An example of a negative contract is the screenshot of Jewelry Store which also has a stealth bonus on it.
You can get a positive boost for the other contracts as well as the chance to get rid of a negative if the contract you plan on completing is not -30% in EXP, but you will have a greater chance of spreading boost by doing heist with positive boost. An example, doing that Diamond store in the screenshot below with 43% EXP boost would spread that boost out randomly to 1 or divide that amount across several contracts after completing the job. (Note: this screenshot was taken before Patch 35 radically reduced the maximum influence of the boosts).
The maximum boost you can obtain for a contract is +15% and again completing boosted contracts will make the negative exp gains from other heist go away much faster. If you always do contracts that have positive boosts you won't go negative for any heist more than likely.

Certain heist grant a stealth bonus that can be combined with the regular EXP boost we already get by stealthing the day or days where stealth is an option. A stealthable heist can be found by looking for a blue ghost icon next to the name of the heist. The lowest possible stealth bonus is 3% and the highest is 25%. Here are the list of all heist that grant a stealth bonus as well as the percentage. It's good to note that difficulty doesn't matter when gaining a stealth bonus. You can play on normal and still get the maximum bonus available.

Note that patch 35 and 36 halved all stealth bonuses.

• Jewelry Store, UK Job, Four Stores, Shadow Raid, Diamond Store: 5%
  
• Art Gallery: 8%
  
• Harvest & Trustee Banks, GO Bank and Nightclub: 10%
  
• Big Oil (Day 1), The Big Bank: 15%
  
• Firestarter: up to 20% total:
  15% for Day 2, 5% for Day 3
  
• Election Day: up to 20% total:
  5% for Day 1, 15% for Day 2 if the right truck was tagged on Day 1.
  
• Framing Frame: up to 25% total:
  7% for Day 1, 3% for Day 2 (mainly depends on Day 1 being stealthed), 15% for Day 3.
  
• All other missions/days officially can't be stealthed and don't give a stealth bonus, even if you manage to stealth it anyway, which is possible in some cases.

The precise mechanism how boosts are redistributed after missions is not entirely clear to us yet. However, we feel confident about some statements:

• If you win a mission with a positive boost, it is decreased to zero (the normal EXP rate). The boosts of some randomly selected missions will be increased. Sometimes, no increase occurs, and we suppose it has to do with having too many positive boosts at that particular moment, but don't know the details
  
• If you win a mission without a boost at all or a negative, the mission's boost may be decreased - sometimes it does not change. Other missions' boosts may be increased. Increases may be applied to several other missions at once. Again, we don't know the details.
  
• Sometimes, after a mission, the game reduces the boost of a contract you have not played by a bit. So far, we have seen this only for missions that had a positive boost, after mission succes and when the total percentage of boosts is quite high, but the actual prerequisites for this effect are not known.
  
• Failing a mission does not seem to alter boosts. This was not thoroughly tested though.
  
• Buying a mission will reset that mission's boost to zero if it as positive. We haven't tested yet if this spreads the bonus to other missions.
  
• After winning a mission the boost decrease on it and the boost increases on other missions do not have to be the same. In other words, the total percentages do not add up to a constant value.
  
• The maximum boost is 15%, the minimum -30%.
  
• Boost changes are being rolled individually for each player in a heist.
  
• It is possible to have zero or positive boosts on all missions at the same time. This has been done by mainly playing missions that grant any amount of positive boost, but we don't know if this works reliably.
  
• The maximum boost can be awarded to only one heist at a time. Then, that heist's icon gets a special glow.
  
• Boost numbers are not affected by gaining infamy (and neither are stealth bonuses).
  
• Boosts are actually proportional to the third power of internal variables that are increased in certain steps. In other words, the boosts changes will become larger, the larger the boost effect already is.
  
• Making a heist negative will still give positive boost to other maps up until that heist hits -30%. We are not certain if it will continue to spread positive boost or change any other map's boost in any way if played at -30%.

If you know more about this, we would love to hear it.

• Stealth boost numbers are blue and boost for doing different heist is Orange for positive. Red for negative or EXP reduction.
  
• Stealth and boost bonuses are multiplicative, and the maximum boost obtainable by using them is +43.75%. Getting it would require you to fully stealth a Framing Frame job. This would get you a 25% stealth boost for your next heist on top of the boost if any already applied to whatever heist you choose.
  
• Conversely, if you have a good stealth bonus, try to play a mission that gives good EXP, like the 3-day Pro Jobs on Overkill.
  
• Since Buying contracts resets their boosts, you should not do this if you want to take advantage of them!
  
• Gage packages give a little bit of extra EXP depending on map and difficulty. It is generally not worth the time investment to hunt for packages (if you do it for the EXP). Gage Packages give up to 5% exp if you collect all of them. Comparing the exp bonus to Infamy I bonus shows that the exp bonus is exactly the same. Was tested on both Hard, Very Hard and Overkill heists.
  
• Infamy bonuses do NOT reduce the amount of EXP needed to advance. They do, however, add a bonus to EXP gained. This used to be wrong in the in-game description but has been corrected.
  
• Making a heist go into negative will still spread positive boost to others up until -30%. This can be useful for heists that can be finished very quickly at lower difficulties such as Jewelry Store, Ukrainian Job or Four Stores.
  
• Quite obviously, always use a perk tree with the 4th perk, "Blending In" unlocked. So if you don't have it yet, concentrate on one perk tree to get it quickly, then use it to unlock everything else you want. If you already have a fairly complete perk tree, the first three bonuses from another tree almost never justify taking it for a mission.
  
  
  Our recommendations
• "Surfing the wave of positive boosts" by always playing one of the missions with the highest boosts is a very viable strategy for leveling quickly.
  
• Banks on Death Wish are easy to pull of with one or two semi-competent ghosts, but pay nicely, both cash and EXP. This is not the fastest way, but a very laid back one.
  
• If you have a competent team of heisters that is willing to re-spec, the Armored Transport missions on Overkill can be done in under 5 minutes with two people equipping C4, one with a saw and one with Inside Man (to buy the expert driver). In terms of EXP, they pay about the same as banks, but can be done in half the time.
  
• For advanced ghosts, Shadow Raid on Death Wish are still very manageable with good EXP and excellent cash payout.
  
  
  Note: Encouraging players to play varied heists was the stated goal of the boost system (introduced in patch 24 with the Death Wish difficulty) in the first place. Unfortunately, base EXP rates were so unevenly distributed that some missions and sequences of missions were clearly favored for leveling up. Finally, patch 35 re-balanced base EXP rates to such a degree that the desired playstyle was actually competetive to grinding only a few missions.