Retarder and paints with lots of water are the best methods I\'ve found. Also, make sure you\'re using a liquid retarder and not a gel one.
I prefer to layer to blend, so I am not an expert, but the best way I have found to do wet blending when I was experimenting was to use a ceramic pallet with wells. I would put in a few drops of paint, a similar amount of water and a drop of retarder. Mix well. Each well would have one of the three colors (Wet blending I use three colors, the shade, the base and the highlight - if you\'re just trying to blend two colors, for example making a red to purple gradient than you can use two colors and the lighter color as the basecoat). The model would already be basecoated before I began the actual blending. Place the two colors you wish to blend side by side on the model. Either quickly wash out your brush to do the blending or keep a separate wet brush to do the blending.
Be sure to do only small sections of the total at one time, try to do too much and you won\'t have time before it dries.
Hope this helps. Wet blending is not my usual way of doing my shades and highlights (I use layering most of the time) but that\'s what I know of it... if done correctly you can get pretty smooth blends in a lot less time than layering, but it also is MUCH harder to do well IMHO.
Edited to add: There\'s a tutorial in the articles section here: http://www.coolminiornot.com/article/aid/58
