I realize some guys here may not even know what a presale is. I participated in one most recently with Civic.  Sometimes they come with special discounts that people at the ICO can't get, but in Civic's case, you paid the same price regardless except that you could buy an amount of tokens that you would not be allowed to buy at the ICO.  The "minimum" for participating in Civic's presale was $50,000 worth of BTC or ETH.  For the ICO, the "maximum" you could spend was $25,000 worth.

In the case of Civic, both presale and ICO participants were able to get tokens at $0.10.  This is actually the same price that early investors in the traditional equity round paid (pre presale people) which is unbelievably fair. 

Every presale has different terms, and not every project announces presales publicly like Civic and Sia have.  Sometimes you have to e-mail the project and ask.  On many projects, presales can take place a few months before the actual ICO.  In those cases, there is a lag between the time you contribute and when you get to tokens, to when they list on the exchange. You'll have to decide for yourself whether it's worth the wait. Whether or not the project has incentives in the form of discounts or additional tokens for participating in the presale is one criteria to consider.

For those of you who may not have the minimum funds to do a presale, maybe consider a pool with friends or people you trust.  You guys can combine to meet the minimum for the presale.