I think the most important part for building Lego Transformers is starting with a good idea for a transformation step. Not the whole process, just a small little part, that could develop in multiple ways.
Both of my favourites (>>7832793
) were made that way. The Spyrius one is based on the idea to use the hinges (left side of pic related) to collapse the long body of a plane or starfighter into a more compact shape usable for humanoids. For the other one I started with the idea to have the two large engines in the back be folded up feet.
Starting with that thought process, at least for me, results in a more "organic" feeling transformation and helps to have both modes "fit" each other.
A contrast to that is >>7832801
, who I made to play around more with articulation. This resulted in an overly complex transformation and something I call "accordion syndrome", where there are so many hinged and movable parts, that you can essentially make unlimited modes, but none of them will look really clean. Also I payed no attention to where the arms or legs would wind up, which caused problems later on and is a reason for why the transformation and altmode are quite messy.>>7832795 and >>7832797 were built with a different approach, where I started with an altmode in mind. They worked out pretty well, but I think there was a good bit of luck involved. I think starting with a rough transformation idea yields far better and more reliable results (Of the few ideas I scrapped, all started with an altmode idea).
As for general tips, get a small little box, fill it with all kinds of small hinges, connectors and clips (especially the new rounded ones in pic related are great) and just grab a handful and fiddle around with them until you get a good idea what a certain function may be useful for.
And there's a few pretty cool mocs on Youtube and a lot of them even show instructions, which really helps to get inspiration as well.