I have four modes of shuffling:
- 1-5 cards: mix them up a bit then ask a neighbour to do likewise because I literally can't shuffle so few cards without knowing what's where.
- 6-20 cards: multiple overhand weaves
- 21-75 cards: riffle shuffles
- 76+ cards: riffle shuffle sub-decks, regularly interchanging halves between them
Fortunately, that last one is for other games and very seldom comes up in Dominion. (-8
I'm aware of the research which suggests seven riffles for a 52-card deck, and without evidence extrapolate this to riffling approximately an n-card deck approximately sqrt(n) times. When I've had to split the deck up into sub-decks, I make sure each card has been riffled sqrt(n) times.
I tend to cut about 1/4 of the cards between shuffles, just to mix things up a little more quickly.
In practice, when people are playing slowly, I tend to continue shuffling until the player to my right enters their buy phase.
I'm satisfied this is all completely fair, unless using cards from the first printing of Adventures, at which point I subliminally control them to the top of my deck. )-8
(When playing Bridge, I use a dealing sheet - multiple lines, each line consisting of the digits 1 to 4 appearing 13 times each in a properly random order. You roughly shuffle the cards so people don't know what's where, deal according to the topmost unused line, then cross that line out. This is quicker, easier and better than normal shuffling.)