Sandcastles in the Sand

There are a couple different things you can do to a model's base.  The easiest is to just paint it green, like the Human Blitzer below on the left:

All his base are belong to me.

It's pretty simple and straightforward; glue the player, paint, done.  And it looks like ass, especially compared to the High Elf Thrower on the right. 
Getting that look is pretty easy, so no reason not to do it right.

I spread normal paper glue across the base.  I'm using a skewer, you could also use a toothpick, matchstick, even an old paintbrush.  It doesn't matter as long as it gets spread evenly and as little glue as possible gets on the figure itself.  And if there's a gap in the base (like the ones on the side on the human above), just slather glue over it.  It's thick enough to cover up all but the largest gaps (which I cover with scotch tape).
In the US, I use white Elmer's glue.  Here in Germany the glue I'm using is clear, so it's hard to see on that base, but it's nothing special, it's the same completely normal inexpensive glue that you'd let a 5 year old use.

Once the glue is even, dip it into a container of sand!

This finer the sand, the neater the grass is going to look.  This is going to come out looking like a short-cut, well tended lawn (with a rock on it, this is Blood Bowl!).  Coarser sand will give you a rougher look, like the High Elf above.

This goes pretty quickly.  I had the entire team done in no time.