These are tips cameron wrote up to be CAMPAIGN on expert through co-op. http://brandpropel.com/l4d-expert-tips.html
I suggest giving them a read. Not all of them apply to versus, but most do.

Be sure to read the new patch notes, go to games an right click on it to 'View Update News'. They changed quite a few things that will affect strategy, especially as infected. Seems like, hunter and smoker are better now.