The overhand shuffle works in almost all cases although very small cards can be annoying. The ultimate fallback is the pile shuffle where you just put cards in to multiple piles then merge and repeat if you feel the need. That technique is useful for the rare cases where you have to randomise tiles or other non-card based elements.
1. Random pile shuffle, dealing "manually randomly" (i.e. arbitrarily in no particular order, not in a fixed repetitive cyclic sequence, but even though I explicitly say this, probably someone will object that pile shuffle is completely deterministic and cyclic and bad...) into a half dozen piles and then recombine the piles in some random/arbitrary order.
2. Overhand shuffles peeling a few cards randomly from the top and bottom repeatedly, using a technique (that is second nature to me but that is apparently uncommon) of repeatedly pulling the top few and bottom few cards from the deck in my hand and transferring them into a newly forming deck, i.e. if the deck is like this in my hand, then a few steps would go:
As already mentioned, overhand is generally preferred. A few light, slow, deliberate riffle shuffles are okay. And pile shuffling is good for those new/stiff/big/small cards. I usually then overhand those stacks a few times as I add them to the bigger stack.
But if they're sleeved, just mash em all together!
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Riffle shuffles don't have to bend the cards. This takes some practice, but I've seen people with very small hands shuffle big decks. What this video doesn't show is that you can bevel just the cards about to be shuffled, with any number of cards above the bevel.
All reality is a game. Physics at its most fundamental, the very fabric of our universe, results directly from the interaction of certain fairly simple rules, and chance... (Iain Banks)
Small cards are not a problem with practice. Really big cards are difficult (Inis!), as are very large numbers of cards (Deus).
For oversize cards, divide them into packets then pile shuffle while gently cursing the publisher.
If it is a biiig deck, divide it into manageable sub-decks by dealing packets into several piles. For each pile, alternate riffle shuffles with over/underhand shuffles. Merge all the cards into one big deck once more.
If it's inappropriate for a rifle shuffle, the surest way to randomize the cards is the tried and true "pile shuffle" (divide the deck into seperate stacks, such as 5) with a various Indian shuffles and finally a half-deck cut. I don't think doing Indian shuffles is effective by itself since it still clumps cards together.