It's also a good idea to check the printer settings for the laser printer. Many printers have things like Eco modes, some resolution "enhancement" stuff, etc. Make sure that all this stuff is turned off. Use the highest resolution setting, and select the darkest setting (i.e. most toner applied during printing) if the printer/driver allows for that.
If the layout is not too fine, you can stack two prints on top of each other. However, if you want to do finer layouts, this will cause problems, since the topmost sheet is some distance away from the photosensitive coating, and will cast shadows.
Generally it's not that important if stuff like ground planes or very thick traces look slightly spotty, at least not with semi-transparent paper. Oh, and i guess you know that already, make sure that the layout is mirrored, that is, the printed side of the sheet is in contact with the PCB.
Greetings,
Chris