Tip #6 - When one or more players have called in front of you, you need a big hand to raise
When one or more players have already called in front of you, you need a much stronger hand to raise than you would need to open the pot. This is because the callers have already demonstrated some strength by entering the pot, and you must be prepared for the possibility that one of them will reraise.
For example, suppose you are in middle position with A-J offsuit. If everyone has folded to you, this hand is worth a raise. However, if a tight player in early position has already called, you should probably just call as well. The early position player likely has a strong hand, since tight players don't enter pots from early position with weak hands. Your A-J could easily be dominated by hands such as A-K or A-Q.
Another consideration is that when players have already called in front of you, they are getting better pot odds to call your raise. For instance, if two players have called $10 each, there is $25 in the pot ($10 + $10 + $5 small blind). If you raise to $35, they each need to call only $25 more to win a pot that will contain $120 if they both call. These are attractive odds that will often justify a call with hands that might have been folded to an opening raise.
The hands that play well in this situation are pairs and high cards that are less likely to be dominated. For example, K-Q suited is a better raising hand than A-J offsuit when players have already called, since K-Q is less likely to be dominated than A-J. Big pairs such as Q-Q or better should always be raised in this spot, as should A-K suited.
Remember that when you raise after others have called, you are announcing that you have a very strong hand. Make sure you actually have one before making this declaration.