What's a good way to shuffle small decks of cards?
The pile shuffle recommended by Jonathan Hobbs works OK, although I'd strongly recommend randomly varying the order in which you deal out the cards to make it more random.

The main issue with it is that the randomness comes almost entirely from the shuffler's choices, rather than from uncontrollable physical randomness as in a riffle or mash shuffle (not that those can't be done "perfectly" too, but it takes a lot more skill).

A variant that you can do in your hand is the Mongean shuffle, where you hold the source deck in one hand and pick cards from it alternately to the top and bottom of the target deck held in the other hand.  With a bit of practice, you can do this really quickly and efficiently.

Like the pile shuffle, the Mongean shuffle doesn't actually introduce any randomness if you do it perfectly in consistent order.  However, you can break up the order by sometimes dealing more than one card in a row to the same end of the deck; if you do this perfectly randomly (i.e. switching ends with 50% probability after each card), the Mongean shuffle actually becomes about as good as the theoretical random riffle shuffle.  Of course, again, the source of the randomness is under the shuffler's control, so a skilled shuffler could, in principle, use this to stack the deck.

A nice feature of both the pile and the Mongean shuffles is that, even if done non-randomly, they still serve to spread out clumps of cards; indeed, a "perfect" Mongean or pile shuffle breaks up clumps more efficiently than one would expect by random chance.  For some purposes, this may be sufficient.

Finally, if you want to shuffle a small number of cards really randomly, one of the most efficient methods may be the simplest: the wash shuffle, where you simply spread the cards on the table face down and push them around randomly before gathering them up again.  It's simple, it's crude, but it does the job.