Ah, ya my big tip is NEVER mash. If you mash the move will always come out slow. All of Bison's trials are pokes which are those annoying light kick/punch combos that have rediculously strict timing. There really isn't much to Bison's trials really. If there is a secret it's just, if they block, too late, if the move doesn't even execute, to early. His timing for moves is basically right on the milisecond so you just have to get a fairly good rhythm going when you use the combo. Keep at it though. Bison's like I said are just luck/timing things.
Part 2 ;P Now if you really want to 'cheat' you can use the pause method which helped me a BUNCH on Viper and Gen and it made the challenging trials like Cammy become absolute cakewalk. What you do is when your at a challenging part, execute part of the combo and immediately after a button press, hit pause. Now let's say you need to focus dash out of a move followed by a very fast QCF P for something else. Do the first move, pause immediately. Unpause and quickly hit the focus buttons and wait until you see him charge and pause. Unpause and execute the rest of the moves. This takes trial and error but it's a pretty fast method to master and when it does you will eliminate a LOT of the trials. It works for almost everything.
Unfortunately that's about all of the tips I can give you, if you are still struggling tell me a specific character and I will try to help you. Hope this helps. I'm considering making a Youtube video about the pause method.
Hey uh I got your post and I had a couple ideas of ways to improve your abilities for comboing in those hard trials. The first thing I did was go for all of the easy guys. That way you don't feel overwhelmed by the number of trials you have to do in the first place. Definately start with M. Bison, all of his trials are poking combos basically. A couple of 'easier' trials are Zangief, Honda, and from there you should start getting the hang of the trials.
Obviously all of the trials are based on timing so heres a big key and I don't know if you know this or not but it helps anyways: If the move doesn't come out, your too fast, if he blocks it, too slow. The timing between these can be on a slight milisecond so it's just basically keep trying until you execute it.