Litko is the most common recommendation I've seen, eg:
http://www.litkoaero.com/page/LAI/PROD/TS/TS027-SM
Or if you want to have another go at your own see dropship here:
http://www.dropship.org.uk/epic/ter001.asp
I made mine as follows.
50 blast markers done using the steel wool technique. It is possible to sculpt artful mushroom clouds and explosions with bits flying everywhere, but I wanted lots done fast! Technique used took about 2 hours and involved the following:
1. Tufts of steel wool glued to counters, and then sprayed flat black.
2. Edge of base painted in earthy colours (don't do this if you want to use them for BFG as well!).
3. Bottom of explosion drybrushed white, then white drybrushed yellow (white helps bring out the yellow but it's not really necessary).
4. Red drybrushed over yellow with more red concentrated towards the top (note many people get this the wrong way around - flames are red on top and yellow and white below - so to paint flames you reverse highlight).
The sequence of painting the BM can be seen in the lower right of the photo:
