The trick is just to cut them perfectly every time. 😉 Yours look pretty good, so bit of practice and you won't need to worry about anything other than minor gaps soon (And the occasional large scale mess up).
For small gaps I add a bit of glue first, then lightly sand to fill with sawdust instead of adding sawdust then glue. It seems to hide it slightly better than adding sawdust first then glue, well, that and the sawdust always comes from the wood you are matching, so colour variations are less.
No jigs, but I always mark the waste, I've cut away the wrong bit too many times.
