This is a great question because the natural tendency after you get a treatment to stop wrinkling is to try to help it out by not creating the wrinkles by frowning or squinting, but have no fear! You can't (for better or worse) make the Botox wear off faster by engaging the muscles more, and you can't make it work less well by wrinkling the skin by frowning or squinting. It will take two weeks for the Botox to take its maximum effect and holding your facial expressions flat during this time will not help it turn out better. In fact, there are some doctors who feel that engaging the muscles of expression early on after treatment helps the Botox get absorbed into the relevant muscle more effectively, making the results even better! So squint away.... you may actually be helping your results!
On a side note, if your eyes are very sensitive to sunlight, please do wear protective sunglasses that block UV rays on a regular basis. Not only can you get skin cancer on your eyelids from excessive sun exposure, but you can get melanoma skin cancer on the retina in the back of your eye, as well as increase your chance of macular degeneration and blindness later in life with excessive exposure to very bright light. If Botox weakens your ability to squint your eyes shut in a normal, protective way, this is even more important.