Always always prep!
Regardless of what paint you're using, the prep is necessary. Don't fall for the "No Prep" gimmicks. I love to use Frenchic's Sugar Soap to clean my pieces before painting. It gets rid of all of the dirt, grime, and years of built up residue. Just spray it on, wipe it down. If it's a really dirty piece, use a scrubbing sponge and really get in there! You can also use "totally AWESOME" from the dollar store or krud kutter. They are both a good choice for furniture cleaning. Once the piece is clean and dry, make any necessary repairs (I filled the knob holes) and do a scuff sand over the entire piece using 220 grit sandpaper. This keys the surface and gives the paint something to adhere really well to. Then wipe down with plain water to remove any dust and residue.