Haven't tried it, but how about glueing some of your aluminum to the bottom of each IC in between the legs with some thermal epoxy. It could extend about 1/2 inch beyond each IC.
Yes you can use heat sinks or just stack more chips.
For heat sinks just solder some copper wire to the center ground pins.
Use sn754410 for higher currents than the l293d.
Look hear; http://letsmakerobots.com/node/26101
Has I said it is a arduino motor shield already made, so i have to stick with what is already on the board, so just stacking another L293d will give just the safety current I need in case I got a motor stall, and the heat shink was just in case they come hot a little bit.
so after few toughts, i was heading for some sortof 2 pieces of aluminum glue on top on each ic, but after rereading the datasheet Iwill probably go for some sortof 2 mounted pieces on the ground pins each side, who should give me better heat transfert ... i dont think they gonna be very hot anyway ...
but after rereading the datasheet Iwill probably go for some sortof 2 mounted pieces on the ground pins each side, who should give me better heat transfert