Keep your pencil sharp at all times. I'm sure you've heard this already but it makes a big difference. You can see a difference in the overall clarity here, I used a dull pencil before and then kept my pencil sharp after.
Buy templates and use them even on quick 5-10 minutes sketch like the one above. The more you use them, the more comfortable you will get with them and thus improve your perspective. Here are the tools I used:
Take your time but move quickly or "work smarter", I often make the mistake of thinking that I have to do a sketch quickly for it to look energetic. I find that makes it sloppy. Now I try to think out the sketch, draw some basic construction lines, get the right eclipse/perspective, then use quick stokes to apply details. That gives the "fast" look... and it takes about the same amount of time.
Finally, PRACTICE... Which I need to do more of.