You could always just use wax. If you are going with polyurethane, firstly make sure it's oil-based, as there seem to be a lot of poorly-labelled "water-based polyurethane" varnishes around these days, which are almost universally crap. Secondly, wipe the surface with white spirit before applying the first coat, and use a good brush, not a spray. And keep the coats thin. Three or four thin coats are much better than a couple of thick ones.