There's not much you can do. However, if you get a weak signal, you'll find that positioning the laptop and rotating it within the room can sometimes make the most of a crappy signal. If you have anything to monitor the signal strength or SNR you'll find it helpful (netstumbler works, and there are many others).
If you want to consider adding a card, I'm a big fan of the SMC 2532W-B high power 802.11b card. It's about $55. Great sensitivity and you can add an external antenna too. If you're doing business travel just expense it!
