I always use quality gaskets and a good sealer. I've found cheaping out on gaskets and sealer is just asking for leaks.
I use this stuff for sealer:
Heres a Jegs link:
http://www.jegs.com/i/Permatex/775/85420/10002/-1
I've found it at AZ and with the part number it should be available wherever Permatex products are sold. I have found some places don't carry it. I think this may be that it is expensive so it sits on the shelf longer than the cheaper stuff. It's also been my experience that stores usually only stock what sells faster.
It's equivalent to hylomar (which works excellent). Like the "Aviation" sealant alsriv2 mentions in their post it's non hardening, stays pliable, fills voids. It may even be the same stuff as I believe hylomar is also widely used in the aviation industry. Old gaskets (even thin paper and cork) are easy to clean off when their sealed with this stuff, they peel right off, usually in one piece. No more scraping with a razor blade or gasket scraper, saves on expensive "gasket remover" spray too. Sealant residue wipes right off with acetone, brake cleaner, carb cleaner etc... again no more scraping. It's not silicone based either so it's safe for sensitive emission sensors also.
I'm unable to recall ever having even a small leak since I started using this type sealer about 20 years ago. I have even disassembled components and reused some old gaskets when I've used this. Great for checking valve clearances. It's impervious to lubricants, coolant etc... I apply a thin film on both sides of the entire gasket sealing surface. Seals instantly, no need to wait to fill with fluids.
I don't own stock or interest in the company either, I just like the product as it's made my life a lot easier.
As previously mentioned in GASoline71's reply, I also make sure the pan rail sealing surfaces are straight. I remove all previous over-tighting warpage and damage with a hammer and dolly and then only tighten to the correct torque with a proper torque wrench. I've found if gaskets are not torqued properly but rather by "feel" then gasket failure is very likely.
I also check for cracks or pinholes. Some of them little leakers from actual pan damage can be pretty small and easy to miss. I'll usually clean the inside surface with a fast drying solvent like brake cleaner or acetone. Then apply a penetrating oil (WD40, PB Blaster etc...) to the opposite side and then examine the "cleaned" side carefully for penetrant seepage. I use the non-aerosol WD40 applied with a brush for this and if needed a little added blue or red Dykem to the penetrant makes it easier to see seepage. Any Dykem residue cleans right off with solvent.
I will usually apply torque in 2 or 3 steps as I've found it helps prevent creasing, bunching or other deformation issues with gaskets. For example if an assembly requires 21 ft. lbs. I will torque to 7 ft. lbs., then 15, then the final 21 ft. lbs. value.
IMHO cork gaskets are OK but they relax after being tightened down. Cork gaskets need to be torqued several times before they will stop relaxing and seal properly. I always torque them down, wait a few minutes, torque them down again, wait again and keep repeating this until the gasket finely stops relaxing and the fasteners remain properly torqued.
With cork I also do the 3 step process but I only do the first 2 steps once and the final value is what I use to do the torque & wait procedure. It's a lot slower to do gaskets in this manner but after learning to do them this way I've had much greater success sealing gaskets the first and every time. I always keep in mind that assembling gaskets this "long" way does take a lot less time (and money) than repeatedly replacing gaskets that fail to seal. I've also found replacing engine oil pan gaskets in some vehicles requires a lot of work and I prefer to fix them in one shot over multiple attempts.
Also when possible, I'll check gasket fastener torques after several temperature cycles. After letting the component reach about 5 - 10 full operating temperature and complete cool down cycles I'll usually recheck for proper torque. This helps prevent fastener loosening from expansion and contraction which will ultimately result in leaks. (This reminds me I need to go check my valve cover gaskets after changing them last week)
Regarding cork gaskets I said they are OK, I have had great success with them. However sealing technology has come a long way since cork was developed and I usually purchase the rubber gaskets when available. I've had better success with them and they can usually be resealed multiple times where as the cork stuff is usually only good for one go around.
Good luck cwilkie with your trans pan leak. I sincerely hope my experiences on this subject can help with your concerns.
Best regards.