How to create a non-slip surface for armor?

It depend from the scale.For 1/35 you can try to use soda.
First mask the area,and spray a heavy coat of gloss enamel varnish.
Apply carefully trying to replicate a smooth coat.
Pull off the tape and leave to fully dry.
Repeat if you want a heavier effect.
Finally spray a coat of color(base coat)
For 1/72 you can use the dust of dry pastels or talcum powder.

i found a website with a great demo on this subject. It mentions Mr Surfacer 500 I looked even more this stuff is pretty cheap too about $5.00 a bottle. Here are some links to the website and a place where you can buy Mr. Surfacer 500. But first read the article as it directly mentions and talks about M1A1 models. It compares all the main brands in 1/35 scale and lists the pros and cons.

http://www.best1hobby.com/
Mr. Surfacer 500 is made by Gunze Sanygo
http://www.greatmodels.com/

Both of the above have Gunze Sanygo, plus alot of other great deals

THis is the website with the M1A1 comparision on the top makers of the tank. At the end of the article you will find the link to the non skid article.
http://vodnik2.republika.pl/pages/m1-comparison/m1comp1.htm