Step 1: Get the items
Get a Crossbow TelosB wireless sensor node--or a Moteiv Tmote Sky. MEMSIC now (July 2010) distributes the open-source Crossbow TelosB boards which is good, Crossbow and Moteiv no longer do.
Get a Datalogger Shield (Adafruit) - in the second picture below it's attached to an Arduino but you're just getting the shield. Buy a 1-2 GB SD-card or a micro-SD card with adapter.
Get an Arduino Mega
Get a waterproof case: we used Pelican 1450 and got a small can of silica desiccant to put inside.
Get a large 6-12 Volt battery. A rechargeable sealed lead acid or deep-cycle marine battery is good. Estimate lifetime: the datalogger uses at most 100 mA at 6V. A 50 Ah 6V battery is good but a 50Ah 6V battery is 26 pounds.
part number OPT-850/6, can be used in any orientation and charged with a 6V "Battery Tender"
This battery should last 500 hours (nearly 3 weeks), more if you carefully write your program to turn off all status LEDs and write to the SD card only occasionally. A 12V battery could also be used with the Arduino Mega. What really happens when it starts to go out is your datalogger turns off and on a lot, probably cycling with the outdoor temperature. Hopefully you collected lots of data before then.
We used hose clamps to connect a power jack to the two large battery terminals. The 2.1 mm power jack for the Arduino Mega came from a wall adapter. Center is positive and outside is negative.
You might also add an external antenna on the TelosB for better reception. Compatible parts available at DigiKey are
SMA connector ACX1233-ND, 30cm cable 602-1066-ND, and antenna A24-HASM-450-ND. While at DigiKey pick up 30 gauge wire, sockets, jumper wires, and a power cord with a 2.1mm jack or a cheap AC adapter for harvesting the power jack.