EDIT: I recommend looking at this method: https://app.box.com/s/cispknq8b9zgog8k5vxn.
I blogged about a WR703N antenna mod earlier, I found some hints about a better way of doing it on the openwrt forum. There's a (0 ohm) shunt resistor in the antenna path - labeled J1. I undid the changes I did earlier, unsoldered the J1 resistor.
With the board oriented with the wired lan to the left, I then soldered the core of the antenna wire to the RIGHT pad of the now removed J1 resistor. There is a big empty pad "north" of J1, where the antenna shield can be soldered.
EDIT: See Diarmaid Ó Cualain comment below, he has been getting better luck leaving J1 bridged (and instead cutting the internal antenna) - while still soldering to the same location. There are two capacitor on the antenna track on the other side of J1, which could be the reason for this.