Over the decades of patching hundreds of tubes on bikes (and cars back in the day) the main reason I have found that patches don't hold is that the inner tube surface was not prepped properly. Those metal scratchers that come in cheap kits don't work. Regular sandpaper is ok. Usually the sandpaper that comes with a good kit is pretty good to (albeit too small though). I use a small swatch of emory paper from a roll of plumbers sandpaper. Hey, it's what I have on hand. The main thing is it's got to be sandpaper. Oh, I don't know, 100 grit maybe? If you have a Dremel tool, the little sandpaper drums are awesome and make for a fast prep job! We used to use an air sander in the old filling station I worked at. Ummm, be careful to not burn through the tube....

Rough up the area all around the hole. That roughed up area needs to be a good bit bigger than the patch one intends to use. Now, rough it up some more. A little more, ....just a few more strokes. Look good? Nice and clean? Darker than the surrounding rubber? Ok, now give it a few more strokes with the sandpaper. Ok, now apply the rubber cement. Use the stuff that comes in the kit or, like me if you're at home, get an 8 oz can of vulcanizing fluid at the auto parts store. It has to be glue that comes in a patch kit or is used for patching tubes. Let glue dry completely (as previously stated and this is important for the reasons given).

Now apply a SECOND thinner coat of glue. Yup. Let dry and slap the patch on and use your pump head or the bottom of a CO2 capsule to really rub the patch on. Sprinkle a little bit of dirt on the patch (or talc if you have it) and mount back on the bike. The dirt and talc keep the patch from adhering to the tire.

Here is another tip that I figured out over the years: If you partially inflate the tube (oh, maybe 1/3 full of air) AFTER you apply the second coat of glue and slap the patch on that inflated tube, the tube tends to not distort so much once it's inflated all the way when on the wheel. I don't notice this on my mountain bike but on my 23c tires on my road bike if I don't do this I do notice sometimes a bump where the patch is. If you put a patch on an un-inflated tube and inflate it, watch what happens. Yep, the tube stretches around the patch. That's what makes that little bump on the tire.

I'll be perfectly honest here though. I use the Park self-sticky patches out on the road. Same deal though, you have to really rough the tube up good or those won't stick for long either.

Why the Park patches instead of regular patches on rides? Because I don't have to worry about a small tube of glue hardening up. I just slap a patch on and go. And as a previous post said, the Park patches do last a long time if put on right.

Sorry for the dissertation on patching but it really isn't that hard as long as you follow the good advice given on this thread. And yes, I buy Rema patches too...the 100 box...because I live in the desert where we have a lot of thorns.