I used the oven technique for my first partial mashes - it works well. If your oven doesn't go as low as 150, preheat it to as low as it will go and then turn it off once you stick the mash in. Your mash temp should stay relatively stable.
I moved from that technique to a mini-mash tun. I followed these directions and it worked great - http://www.homebrewtalk.com/wiki/ind..._Mash_mash_tun
I now brew AG with a larger version of that same build.