I have been using INSTAMAPPER it seems to work well and you can set the interval that it tracks you from about every second to infinity.
I have installed it on both phones I bought and used it to track the wife to try it out, and it even showed as she was circling the parking lot looking for a space at the mall, but it is a super battery hog.
That being said you are supposed to be able to enable it via sms but I haven't tried that yet, that way it wont suck the battery down.
So long story short its on the phones but I have it set so it is in standby, only to be turned on either manually or the untried sms.