Back in the day.... a.k.a. '95 ISDE, I got down to 7 minute tire changes with tubes and about 15 minutes with Mousse bib inserts. Now that I'm back into the casual lifestyle it's more like 20 minutes and a beer. Something I see mentioned here that goes against all I know is the use of grease and/or WD-40 as a tire lubricant. I never use petroleum products on my tires for two reasons: (1) you want something that will evaporate and go away so that you have maximum friction between the tire & rim and (2) many tubes are made of butyl with is not compatible with hydrocarbons - it will swell and get flimsy if it comes in contact with petroleum based grease. Instead I use either a mixture of liquid dishsoap and water or tire mounting lube (a NAPA product called Ru-Glyde). Either of these are really slippery and helpful while mounting the tire, but will evaporate and be gone shortly after your tire is mounted. If there's grease on your tire bead and you get a flat tire, then the chances of the tire spinning on the rim are really high - all you have to prevent that from happening is the rim lock. Alternately, if the tire bead is dry, then it has some friction to the rim and won't spin as readily in the event of a flat.
A helpful thing to remember when changing tires is to take relatively small bites with the tire iron each time. The bigger the bite, the harder it is to lever over the rim. Lots of little bites is easier and faster than fewer large bites. My husband is really successful when he sets his wheel on a 5-gallon bucket to do tires. I tend to fight that program and prefer throwing the wheel down on the floor or even the shop bench. You really have to try a few different approaches and figure what works best for you. Another thing I do is duct tape the rim band down to the rim so you don't have to fight trying to line up the holes for the rim lock & valve stem and duct tape is slipperier than the rubber rim bands (so the tube can slide around a bit on the rim while mounting the tire). If you throw away the rim band and just use duct tape, then tightening your spokes can be difficult as the spoke nipples are taped down and difficult to turn. By leaving the rubber rim band in under the tape you get a really successful setup.
Practice, practice, practice. : )