Take the extra time to close your lines and use the fill tool. If you want to avoid weird corner artifacts use the select outside trick:
1) magic wand everything outside of the area you want filled
2) expand selection by 1-2px
3) invert selection
Also, I tried using sketch-up but be warned there's a learning curve before you can get all your assets made. I kinda gave up halfway through attempting to make a model and just used shift-clicking to get all the straight edges I needed. A lot of my backgrounds and environments are too free-form to really benefit as well, so make sure you ask yourself if the time learning and developing is worthwhile (ie, how likely will you re-use assets, how long would it take for you to just draw out the scene?)
I drew this building like 5 times from slightly different perspectives and with stuff happening to it. Maybe if I were a master modeler the time spent building this wacky building would equal the time I spent drawing, but I'm not lol
The biggest hack is practice, doing wrist exercises, and having fun!