Is it a Juwel? I have the Rekord 80 and I have this problem too, however there is a solution.
Go to you LFS and buy some aquarium sealant. Then go to the kitchen and borrow your mum's best clingfilm. That's right, CLINGFILM!!
Now waddle over to your tank and remove your lid. It may also be helpful to remove an inch or 2 of water, depending on how much free space you have above the water level.
Tear the clingfilm into thin strips and place over the top edge of the glass, so that you get approx 1 inch of clingfilm hanging inside and outside the tank. Do this all the way round the tank, and ensure its smooth and flat to the top of the glass.
Next, get your recently acquired sealant, and squueze it all the way around the inner edge of your lid that sits on the tank. Now leave it for around 15 minutes (some sealants may vary, check your instructions) to go tacky and start to set as we don't want it running into the tank do we! After this 15 minute wait, place your lid onto the tank and press it down lightly. Now leave it for 24 hours (or whatever your sealant's curing time is).
After it has cured, lift your lid, take the clingfilm from the tank and replace your lid.
You now have a perfectly sealed lid that won't leak down the ouside of the glass
I'm about to do this this week so will post pics when I've done it