In my opinion a good base makes or breaks a miniature when on display. I find the bases from Games Workshop are perfect.
I use round 25mm bases for single miniatures, and 60mm "titan bases" for displays. They are available from the games wrokshop
website for $5.00. (56 small round bases per bag and 15 extra large)
For most bases a good heap of static grass available from Woodland Scenics and Citadel Miniatures are a good start. Sometimes
i add a bit of pebbles or talus. To make brick bases i use a saw to cut straight lines across the base and paint it a mix
of 1/2 Guard Red and 1/2 Flat Black and 1/3 Guard Red and 1/3 GW Codex Gray.
For some bases a quick paint is the best solution.
For this X-men base i gave it a few random slashes with an x-acto knife and then painted it silver. When it dried i gave
it a wash of blue then a drybrush of 1/3 silver 2/3 french blue being carefull not to fully cover the dents.
To make the flying figures on the X-men base i cut up pieces of an old nebuliser cord and glued them onto the bottom
of the figures.