This is something im struggling with for one of my tanks (and its painted black aaargh). As for infantry I found that chalks look good. Once the infantry are painted and suitably based use black brown and orange chalks in varying quantities. Look at pictures of mud splatter for the areas where it will gather (or run around in a field for a bit with light coloured trousers on

).
The key is really the base, if it looks suitably urban then infantry weathering is easy. And onto tanks...
Whilst I have had problems with my dark coloured tank light coloured tanks I have found easy to do. Firstly stowage added makes a lot of difference (somehow gives it an urban feel if done right) Next use the same palette of chalks as above brush them into recesses, use some as a sligth wash and dont forget some grey drybrushing. If its a darker colured tank then go for some heavier drybrushing and also add white chalk to the mix for weathering.
As for what type of chalks I've found that my artists chalk pastels are good enough, I think that actual chalk, as in blackboard stuff, might be a bit too coarse but experimenting may find some use. There are actual weathering chalks and pigments out there specifically for this stuff but as far as I remember they are expensive however ive never used them. If you do want to loo for them MIG pigments seem a good bet.
TO add interest to the vehicle scratches (as said above) will help give the feeling of stone damage, a light drybrush with metals can help aswell!
Good luck, hope that helps
A