Why oh why do people persist in using ancient bipolar H-bridge chips such as L293, L298 and SN754410? These chips have high voltage drop, which also means that they get hot and can't take much current.
Probably because they are cheap, easy to obtain, come in thru-hole packages, and are well documented with tons of example designs; in other words, they have real "staying power". The L293/SN74410 works great for most hobby designs at 1 amp per channel (perfect for most small DC motors); the L298 is good for 2 amps per channel, or 4 amps in bridged mode (granted, you need one helluva heatsink for it, though). Even though they aren't the most efficient, for most hobby purposes, both work well.
Used right - the L293 can even be used as a driver for a MOSFET h-bridge (or a larger bipolar bridge)...
My advice is to forget bipolar motor driver chips and use modern mosfet-based chips instead.
If you have links to a good replacement MOSFET-based h-bridge driver chip in a thru-hole package variant that compare to the aforementioned bipolar devices, please post a link or some part numbers - I'm sure that tons of people would appreciate it!
If you really want to use a bipolar chip, then read the datasheet to see how much voltage drop it has, and increase the power supply voltage over and above the motor voltage by that amount.
Can't argue there; honestly, this applies to any and all parts - not just h-bridge ICs...