Prime your models with black spray primer. This is important because black will hide any imperfections in the crevices and deep parts of your model that are hard to paint. If you skip or ignore this step, you get areas that look unpainted and ruin the overall paint job.
Base coat your model with Vallejo Brown Violet (for US Shermans) or Russian Uniform (for British Shermans). Use a large brush and apply the paint in two thinner coats rather than one thick one. You can use the following steps equally well for both colours since the colours are so similar. Paint your tracks in a dirt coloured brown at this stage as well.
Use GW Badab Black wash all over the vehicle.. The reason for this is to add some shading in the recesses around the raised areas and provide some contrast around the details. Also, you want to make sure you don't drench the model in the wash - pay attention to the detailed parts and make sure the paint doesn't pool in the larger flat areas.
Go over any large areas with the base colour if they look splotchy.. When this is dry do a light drybrush with your base colour lightened up a bit with Khaki, beige, or an off white colour. Do this on the raised detail areas and avoid the large flat areas. Also, a light drybrush of GW Boltgun Metal or similar around the track links will help them look a little worn.
Add your decals! If you need to put your decals in a curved spot, try using some decal softener to get them to conform to the area properly. Also, putting down some gloss varnish first where the marking will go will help avoid silvering commonly seen with these types of decals. Also, don't forget the little details like tool handles, stowage, and machine guns.. Additionally, when everything is dry, you can add some weathering by lightly stippling some thin dirt and dust colour paint around the lower half of the vehicle. Use an old brush and try not too go to heavily - remember less is more!
At the end, when you have stuck with the above tips all the way, you should have a well painted Sherman and now can settle-back and relish the rewards of your success. Pat yourself on the back, grab a coffee, and admire your work!
If you stick to the steps above, soon you will be able to churn out painted models at a fast right and complete whole armies at a faster rate. This technique works equally well for half-tracks, jeeps, and trucks, so give it a shot.