The rule is, if they are in the service industry, you should probably tip. For example, if you are getting laser at a salon you should tip. If you are getting laser/electro from health care professionals, then no.
I always tip on hair cuts and waxing, (and really anything at a salon). If you can't afford to tip, or rather cant afford much of a tip, simply letting them know when you tip that you would tip more if you had it to tip, and that you'll try to take better care of them in the future when you have more money. Poor service(or even inability to get an appt) can happen from poor/non-tipping.