to check your wheel: air pressure is easy, you need to keep at least 40 psi plus, check the holes in the rim where the spoke nipples come through, you don't want any damaged eyelet/hole. Turn the bike upside down, spin the wheel gently. Is the wheel centered on the middle line of the bike frame? Keep your eye on the black line in the middle of the rim - Does the black line wooble? if the wheel is not centered or the black line wobbles more than a millimeter, take the bike to a local bike shop (not the guy who did it last time), ask the shop to true your wheel. They may charge you £10 for two but no more than that. Buy a spoke key (£2 at any local bike shop) and ask the guy at the LBS to show you how to do it yourself next time.