hello everyone
how do you connect strings to a wooden marionette in an elegant way, so that the connections will be as invisible as possible?

You need to drill holes through the wood where you want the string attached.  Then thread the line through the hole and out the other side. Tie a not in the line large enough it won't pull through.  For added strength you can add a bit of glue or glue gun to the knot before you pull it back to the opening of the hole.

is there another option? what about places like the head where i can't (or rather not) drill all way to the other side?

Many builders use screw eyes to attach the strings especially on the head. Also you don't have to drill straight through the head you can drill at an angle from front to back on the head.

You can make a drill in the wood and insert a dowel in the hole, as is explained here: http://puppetsandstuff.com/community/index.php/topic,8703.msg80769/topicseen.html# (click to enlarge the picture).
Play with the orientation of the drill, in order to avoid the stress direction of the string does not tend to extract the dowel

thanks! i think i'll drill a little hole (a bit bigger than the eye screw), put a tiny eye screw in, and connect the string

Yes that could work but be careful that you do not drill to deep or your screw-eye well have nothing to bite into. It would be a bit like counter sinking a screw. A bit of glue on the threads of the screw-eye before you start screwing it in can help keep it in place.

that's right, in thin body parts i'll drill to the end and tie a knot.