I'm not so good with combo too, but I think that the practical execution it's not the only factor for it. Often it's also necessary to put the opponent in the mind status for playing exactly like you want and allowing a combo without the risk of holds. I can do some tricks to easy levels (so with the execution of simple moves) but it's not easy, on the opposite I think that it's the hardest thing to accomplish.
Free Step Dodge
Kronin
tokiopewpew
I rather tend to do always the same combos because I know that they will work and I can execute them without any problems. I can try out combos shown in videos a lot in training mode, but as soon as I'm facing a real opponent, I mostly drop them because of excitement, especially those that need to be timed precisely. A bad habbit, so hard to give it up 
Codemaster92163
Most combo videos do provide actual useful combos, though. It's just the "OHKO" or "30 hit stage combo" ones you have to ignore for the most part xD
Saber
Amen to that, Codemaster.
@Kronin: Mindgames are important, but you shouldn't forget the reason for them in the beginning - to get the threshold to a level where you can get decent damage from a launch. And even then, most combo videos don't show max damage juggles per launcher per weight class.
@Kronin: Mindgames are important, but you shouldn't forget the reason for them in the beginning - to get the threshold to a level where you can get decent damage from a launch. And even then, most combo videos don't show max damage juggles per launcher per weight class.
Saber
@Tokyo: DOA's execution is easy as balls though. It does take some practice if you're new to fighting games in general, but you'll get the hang of it. You can always opt for a juggle with a similar damage values but easier timing/execution.