Sew up big rips (I use kite string a big needle and a pair of pliers), cut out the patch & round the corners off. Draw round the patch on the oversuit, so you can easily locate it on the suit when covered with glue.
Rough up the patch and oversuit with sand paper, and most importantly before glueing degrease all surfaces to be glued with Meths or something similar (I had to use schnapps once when nothing else was available-worked ok!).
Apply the glue to both surfaces (being careful to go slightly outside the line drawn on the suit) wait for it to go tacky & stick them together. Place a piece of paper inside the suit under the rip and one over the patch, then stick a heavy weight on top for a few hours.
I have used a few tins of Warmbac PVC adheasive (£5) on tackle sacks/oversuits with this method and none of the patches have ever come off. Last week I also had to repair a rip in my brand new meander Evil or Very Mad using the tube of glue and patch that came with it (sold separately at £1.50) which seems pretty good.
Simon.