Sizing raises: based on pot size, blind size, or what?

[Poker Clif]

I'm confused about knowing how much to raise. I usually follow the rule of always raising 3bb, so that I don't, by varying my raises, give away the strength of my hands. But I recently read somewhere that in addition to the standard 3bb raise, some add one big blind to the raise for each limper that is in the hand. Add to that the fact that some use the pot to size their raises (on the World Poker Tour, Mike Sexton will often talk about a bet being about 1/2 of the pot), and I'm a little confused. Is my "standard" 3bb raise not so standard? Should I be adding to the raise based on the number of limpers? And how does pot size figure in all this? Any insight would be appreciated.