It's kind of tricky, but I've been amazed how small a spot I can paint with my clumsy hands and a round toothpick The tricky part is that you can't expect the paint to flow as it would from a small brush; if you put enough paint on it to make it flow, it will glop off the toothpick in a bead and go everywhere. You have to keep it relatively dry and depend on direct transfer of paint from the point or sides to the spot you want it on. The point or sides MUST touch the spot you're painting, if you've got too much paint for that you've got too much paint on the toothpck, period.
The flat toothpicks with the squared small ends are good too, although obviously they won't be as precise. They are god for stuff like cockpit gauge faces, and I used one recently to paint the green on the MFD screns in 1/72 Hornet cockpit.
BTW, I can't take credit for this tip, it was in FSM a few months ago, and the reader who sent it in says he paints the safety stripes on ejection seats with toothpicks. So, unless he's modelling in 1/32 or larger, it must be possible to put on very small amounts of paint very precisely.