I always do the basing of an army in one go once I have finished painting it. A good base can make an extraordianry difference to the way a figure or element looks. I really do believe that even with a very mediocre paint job, spending a bit of effort on the shields and bases can improve the look significantly. Conversely, a superb paint job can be spoiled by sloppy basing.
I tend to use Basetex, produced by Standard paints for most of my basing. I don't know who retails it I am afraid, I usually pick up a jar when I go to Wargames shows, and never take note of which stand I bought it from! It is esentially a water based paint with grit in it - I guess I could probably make my own, but at £2 a jar, it's just not worth the effort.
I use 2mm thick plasicard for my bases. I have never had a problem with warping or splitting, and it is easy to cut.
Before the figures get stuck onto the base I score the surface, this is to make sure that whatever I am going to stick on there - Basetex or whatever - has something to grip to. I have never tried it without scoring first, but I suspect the surface would be too smooth. I also paint in the edges of the base at this stage, it makes the whole thing look so much tidier when its finished.
Once that is done, I slap big dollops of basetex around the figures using an old tatty brush to work it up to, and onto the figure bases. I use enough to fill the gaps, and even out the overall surface, using a stippling motion to retain the grainy texture. Once that's done, leave it to dry out completely. This will ruin a decent brush, so make sure you use one you don't care about. Stiffer bristles are better.
When it is dry, as in the top left picture, I then dry brush the whole lot with a colour a couple of tones lighter than the Basetex, as shown top right. I then follow up with another dryer dry brushing using a lighter tone. This really accentuates the texture of the base (bottom left). The last stage is the static grass. I put irregular patches of slightly diluted PVA glue on the base - usually including the area around the figure's feet, then scatter the whole lot with static grass. Give the glue long enough to dry - about 10 minutes or so - then tap the excess grass off. A final blow to get the last of the loose stuff off, and there you have it one element totally finished.
Static grass is magic stuff, as I understand it, the synthetic material it is made from builds up a static charge (especially if you shake it before you use it!). When it is sprinkled onto your chosen area, the static causes the strands to repel each other, causing them to stand up. Thus the very realistic grassy look - clever aint it? I get my static grass from model railway shops, it's not too expensive. Games Workshop also sell it, but with an apparrent squillion percent mark-up, my advice is to stick to the model railway shops.
