The only way to successfully prevent a YLOD due to overheating (if it's not too late already) is to disassemble your PS3 and replace the thermal compound. All other suggestions in this thread won't help/make it worse. If you turn it back on before replacing the thermal paste, then your PS3 will almost certainly die very soon.

I suggest you use Arctic Silver 5 thermal paste. It's what I did and it works great (though I did it BEFORE my PS3 ever started acting up).

search on youtube "how to fix YLOD/Ps3"

I have a 40 gig from 2007 also, so far no problems, and It gets used 2-6 hours a day on week days, and a lot more on the weekends, It gets punishment from me and my son, Also its our main DVD player and Blu ray, so Knock on Wood but no problems yet, there are some do it yourself guides on youtube if you want to try to fix yourself, my cousin did it and it worked, its been going strong for over a year now by the YLOD fix it yourself guide, It only took like a hour or 2 to fix also, so take a look at that opction.

I don't want to take the chance of fixing it myself, and ruin it.

I probably won't send it to Sony, because they just fix it, put it in a cupboard and send you a different refurb. One third party repair company said £75+VAT (which comes to about £90), I'm still waiting for a quote from the other.