In many cases the via may be coincident with a component pad. In that case, just solder the component on both sides of the board.
If no component, and you just want to change the track from bottom to top in order to go over bottom tracks, then just arrange coincident holes on both sides of the board and connect a piece of wire through the board.
well, this is how i do it too.
but it is quite slow and i get bored connecting wires from top to bottom. there are some special 'things', i dont know how they are called, but you put them trouch the hole and you press them on both sides and you make a contact between the lines. but it is acold contact. better with a piece of wire and some solder on both sides.