Straight lines

Yeah I just do like Dr Nightmare and paint a little at a time. I make sure the paint it sufficiently watered down, but not too watery that it'll run. You don't want it too thick because it'll look worse, trust me.

I just eyeball it, and if I go too far on one side, I just eliminate the wobbly line by using a bit of the colour on the other side. Sometimes it can be like a dog chasing it's own tail, but you'll get the hang of it and eventually get steadier and steadier, allowing you to paint straighter.

It helps to hold your breath like a sniper taking aim, because your chest expanding can make your arms move.

The best piece of advice though, is something that comes naturally to many, but something simple that others don't do or know about:

Say you're right handed. You hold the figure in you left hand, and paint with your right. You can rest your pinky finger on the figure or your left hand somewhere, to act as an anchor. It dramatically reduces the shakiness.

But you may have known that already.

I got some tamiya tape from the model section at hobby lobby I will try this thank you

The standard way is to tape off sections. Tape comes in all widths, check your art shop by the brushes or pinstriping gear.

Another way is to sculpt the straight sections, making them much easier to paint freehand.

I just freehand them with quick short strokes, if you try to slowly "draw" them youll get squiggly lines, especially with shaky hands.