tea- tree oil - but don't swallow any.
rubbing raw garlic on it - but be careful it can burn the skin around the cold-sore too.
holding ice to it (especially when you feel it just arriving -that tingly buzzy sensation).
putting toothpaste on it ( white toothpaste - not a gel) - i do this at night or around the house - leave a big glump on it overnight and then take a shower in the morning and wash it off - it dries it out.
Of course these all work best in the first few days. Once it has hit full-blown blister stage i think there's not too much you can do, although these all help ease the burn.
there are also over the counter creams that you can get from the pharmacist which are effective too - but obviously they're not home remedies. .
Source(s):
Things i've tried over the the long cold-sore blown years.