Seamgrip, per the above post, is the standard for urethane-coated nylon. Clean surfaces first with alcohol (rubbing alcohol, not the kind you drink). Find a couple of tiny throwaway brushes to spread the sealer along seams. Do it when you can leave the tent set up for a day or two for the seam sealer to dry. I usually sprinkle it afterwards with talc or silt (a.k.a. dirt) if I need to pack the tent when still tacky so that it doesn't adhere to itself. This makes a mess, but then again so does seam sealer all stuck to itself and then yanked loose next time you set up your tent.
The siliconized fabrics used in some very expensive tents use a silicon-based seam sealer. They're ridiculously slippery to water, which helps keep them waterproof, but they're also very slippery to adhesives and sealers. I have to admit I've never used this stuff, since my two Hillebergs have never leaked as they arrived from the factory in eight years or so. That's one of the advantages of buying classy tents for three (or ten) times the price.
Hope that helps.