I like McNette Seam Grip. Simply push the two sides of the tear together, squeeze a little onto it and then spread the seam grip along the tear. Leave it to set and then repeat on the reverse side if you think it needs it (i personally don't).
I've only used this on tears of up to 25mm, but haven't been able to pull the tear open again once set.
In reply to GraMc: I have just read about Tyvek patches in a Patagonia sponsored athlete article which look interesting and were apparently far more durable. I guess they are found in builders merchants but he swore by them. I shall try to find it but wonder if others here have ever heard or used these?
Seam seal tape, as used to er.. seal the seams on waterproofs.
I bought 3 metres for the huge sum of £3.88 in 2004 (still have the invoice in the envelope with the tape) after a recommendation from Captain Paranoia on this very forum.
Irons on (put a tea towel over it so you don't melt it or the waterproofs) but round the ends off first as a square cut is more likely to catch on something.
