> know the tricks for keeping them straight and clean?
Cutting back on the coffee, getting ample sleep and just a shot of Any-malt Scotch can give you just the (straight) edge you need. It's like pouring anything into a narrow-neck bottle.... you can't let yourself think about it too much.
Get a good quality signwriters enamel like One Shot.
Clean the surface first with Bon Ami.
Keep a rag--barely damp with mineral spirits--within reach for a quick removal of any off-strokes and for trimming off the over-strokes at the corners.
Use the best quality striper brush you can find.
Tubes have no edge tend to be tougher than fenders & forks. I've had pretty good luck using a length of aluminum square, U-channel stock(L-channel would work too). Place it on the side of the tube verso the striping but offset enough to provide a fence for guide for your ring or pinky finger. To protect the paint from the metal's sharper edges cover them with a couple lengths of split rubber tubing. This will do the trick as well as keep the guide from slipping around on the slick finish.
Best of luck.