Hard to add much to what Amazin said - a very good outline on auction basics.
One thing to add in conjunction with the 75% rule (some sites go with 78%, btw) - do not be afraid to price-enforce, even with a player you don't really want or aren't targeting.
Using the $20 example, if bidding stops at $13, go to $14, even if you don't want the player or need his stats, etc. Depending on how my auction was going, I may try and push the player to $18. While you're goal is to get 75-78%, it is much harder when other teams are getting players at 70%. For every player you get under value, there must be a corresponding player or players who are overvalue. Letting your opponent get a steal hurts you two ways 1) inflation for the other players goes up and 2) your opponent is much stronger.
Now, if you end up with someone you don't want, but you got him at 70% of value, you have a much stronger team and can use this player (or others) later on in trades.
Which leads to my last point - you MUST trust your values. If players are going for $3-5 above what you have them valued at, you have to let most of them go. Getting 1 or 2 if needed at a key position or for certain stats is fine. But consistently paying over your value is a sure way to lose. You have to trust that if your opponents are overpaying based on your values, then the time will come when you'll be able to get other players at less than you value. $3120 of salary will be used to buy $3120 in value (12 team league, $260 cap). For every dollar spent above value for a player, another player can be had for a dollar less than value.