I feel your pain - straps (and worse - buckles!) are a very time-consuming part of paintjobs on many models I find.
What I do is similar to Hammers.:
1) Paint the area in a dark colour first, making sure that you cover the "thickness" of the strap as well.
2) Paint along the strap carefully with a colour that's between a mid-tone and a highlight.
3) Give the area a careful wash with a suitable colour. This lets the dark colour show through in the recesses, darkens the colour you painted on in step 2, and provides a nice graduation for you.
4) Pick out a few highlights here and there, concentrating on just the upper edge of the strap (if horizontal), or along the centre (if vertical).
You can stop at the end of step 3 (I often do!), and for anything very small you can even stop at the end of step 2. Really, step 4 is a finishing touch if the straps happen to be a prominent part of the model, or something that's easy to reach like a belt or shoulder strap.
I would also make another observation too - painting straps is rather like painting freehand designs in that you need a decently-sized brush with a very good point. The size of the brush ensures that your paints wet as you apply it, and it's the sharp point that lets you get a precise and neat finish.
Even if it's just for occasional use, I would recommend getting yourself to art art shop to have a look at buying a good sable brush for this. I am lazy and use W&N Series 7 Size 1 because they are fairly easy for me to buy. However, there are lots of other brands that are just as good or better (and probably cheaper prices too!).
Edit: basically I do the exact same thing as Malebolgia it seems!
