When something says you want to steep this @ 160 degrees for 30 minutes how do you keep steady at said temp.? I guess the same question applies to mashing when I get there...
For steeping, you can 'ride' the stove control, or you could put the pot in the oven if your oven control goes that low. I'm sure there are other ways too. For full scale mashes, the cheapest/easiest reliable approach IMO is to build a mash tun out of an insulated cooler.
Stick a thermometer into your steep, when it cools off a little, put a little heat under it and stir. Note that the temp is not critical for a steep, as long as you're somewhere in the range of about 150-165 it doesn't reall matter. Mashing is another issue, temps here are more critical, as Vike mentioned some mash in a cooler, some use a kettle that they can apply direct heat to.
You could build a mash tun out of an igloo cooler since you'll probably move to all grain brewing at some point anyway. However, don't worry too much about temperature for "steeping grains". You're getting all of your fermentables from your dry or liquid extract. The grains are most likely not fermentable nor do you really care. You're simply using them to extract color, some flavor and body. I used to do this. Heat to about 165 F, kill your burner and add your grains (assume you have them in a grain bag). Put your boil kettle lid on. Monitor your temp... you should be fine for the entire 30 minutes. If your temp starts to fall too much below 160... then fire your burner back up and heat to around 165... You'll really be fine for steeping anywhere between 155 - 168. Don't go above 168-170 though... this can start to impart tannins / bitter off flavors in your beer. Again, don't sweat it too much for steeping. Now if you're doing all grain or a mini-mash... temperature starts to definitely matter. Hope this helps.
In my experience, if you're using a couple gallons or more of eternal, it will normally hold the temp, or close enough for government work, for the half hour or so you need. Especially if you put the cover on the pot for that time. Even for a full size mash, I did a brew in a bucket and only needed to add an extra quart of boiling to keep it within range for the full hour. I will be building a cooler mash tun for the next breed, though.
I just get the temp of the water to around 165f. Once at that temp, I add the grains to the water in a muslin bag and let it sit for 30min or so. That's it. I don't goose the heat or anything, as others have said it isn't that critical.
