Seam Sealing
Although most modern tent sewing techniques use threads which expand and fill any stitching holes when wet, most tent manufacturers cover themseleves by suggesting that you seam seal your shelter prior to use in potentially heavy rain. This is fairly easy to do and just requires a warm dry day (possibly the hard part!).
This is the way I do it
If the design will allow it, invert the tent and pitch it inside out, so the seams are slightly under tension. Let the material get warm in the glorious sunshine! Get your tube of seam sealant (most outdoor shops sell it) and squirt a little onto the bottom of a glass jam jar. Then using a children's cheap stiff stipple painting brush, dab it in the sealant and stipple it along the seams, a small amount at a time.
It isn't a quick job, but done this way it is much neater than squirting sealant all over the place which leads to unsightly blobs all over the material.
Then let the finished job dry in the sunshine for an hour or so and when completly dry, you can pack the tent away and the material won't stick together.