Here is a quick waxing kit that you can use before a race, or regularly for training. Total cost is about 5$ and it lasts a season or more.
An issue with this type of quick waxing is that it traps dirt under the wax, if the snow was dirty... but my thinking is that the wax will wear off anyways. To be sure though, you could get a brass brush and quickly brush the base before waxing - that is part of hot waxing++.
See Razie's blog for more Ski Tuning Ideas.
P.S. Keep in mind that the wax won't last too long with rub-on. The best is to hot wax it. The next thing in the hierarchy of easiest and cheapest is to use a heat gun.