it depends on the deck i am using. some are quite large, and I have small hands, so shuffling them can be difficult. in that case, I break them down into smaller piles to shuffle, or place them face down on a large table and push them around for awhile to shuffle them.
other decks that I can handle, I shuffle by holding them on their long side and shuffling them into a new pile. I do this 21 times, then divide them into 3 piles and pick those piles up, but one pile will be turned around the opposite way--thus I can end up with reversed cards. then, I repeat this two more times. when I'm ready to begin my 'reading' for a client, I concentrate on their question or them (for a general read) and shuffle again 7 times. or-if they are in front of me, I have them shuffle the cards till they feel it is enough, while they concentrate on their question.
when I am reading with oracles that have 52 or less cards in them--such as the playing cards, I will shuffle by riffling them. as long as they are the same size as a deck of playing cards. I still follow the 21 shuffle rule tho-to make sure they are good and mixed!
I have a few decks that are rounded, and they are not easy to shuffle--the tea leaf one has over 200 cards in it!
so, they are a deck that I place down on top of a large table and I move them around and around, to mix them up. they are also rather thin cards, so shuffling them can easy damage them--and they are not easy to replace, being an OOP copy. I only read with these for my clients who are not present for their reading, as I don't want them to be handled by clients.
these are my 'SPECIAL CARDS', but they are rather accurate! so, special handling is vital--especially with the shuffling!