What a coincidence! We actually covered this topic at a Queensland Sports Podiatry Group (QSPG) meeting earlier this year... Blisters are a fairly common sighting in the QSPG tent post-race at events like the Bridge to Brisbane and Brisbane Marathon!
Unfortunately, we only ever see the participants after the race when the blisters have already occurred, and not everyone thinks to take precautions against blisters. It seems to be the ones who have brand new shoes (and haven't tried running in them yet) that suffer the worst!
The main guidelines that were outlined at our meeting:
For blisters that still have their "roof":
irrigate or swab the blister and surrounds with chlorhexidine/alcowipes then use a sterile (of course) scalpel to lance the bottom corner of the blister. This allows drainage using gravity. Once the blister has drained, apply a liquid antiseptic, eg. Betadine, and cover with a sterile non stick film dressing.
For blisters that have burst or lost their "roof":
irrigate the area with chlorhexidine solution and if the roof of the blister is displaced but still attached, carefully move it back to cover the blister floor (acts as a biological barrier and helps protect the wound), and apply antiseptic and sterile dressing. If the roof of the blister has come off completely, a protective dressing such as Allevyn (or similar, I think this is what we have in our group's First Aid Kit) can be used.
Deflective felt padding can be very useful to protect the blister site and reduce pain levels. It is a good idea to draw a rough outline of the blister on the outside of the dressing pad (before adhering it to the patient! :p ) so that you can use it as a reference point when you go to place the deflective padding after.
I hope this info helps a little...
these are the main guidleines we use at here at QSPG events... but what do others do with their blister patients?