I use the standard 3M sanding blocks, as found in your local hardware or big boxy store, quite a bit:


I know they're not always as flat as they could be, but they get the job done for me, and it's easy to install sandpaper on them.
I acquired the little green 3M sanding block at the left at a recent sale (sorry for the fuzzy picture; don't know what happened), and own a couple of the Warner blocks, to the right, and use them quite a bit too:

You can see the profile of the Warner block, useful for some molding and other tight-spot sanding:

If ever I get around to removing rust from some of my hand saws, I may dedicate one of the Warner blocks to that: nice flat wooden "platen."