Jaywalker try a gel top mattress.

Freethinker, thanks - fine idea. I'd never heard of that before, nut I looked at geltops today and it seemed to work fine. I'm a tire-kicker from way back, so now I'm trying to figure out why.

This was never a problem for me before they started putting foam in the mattresses.

Usually a Latex mattress is cooler than a conventional foam mattress. I spend almost 6 months out of a year in SE Asia, where its hot and humid all the time. I find a 100% latex mattress cooler, Anytime you make a soft topper it will be warmer, because your sinking into the mattress more which keeps air circulation down. The gel tops and the gel-infused memory foam advertise to be cooler but it really comes down to simple fact that more air circulation will make a cooler mattress.
A spring mattress with a soft latex top maybe your coolest mattress.

Jaywalker, I wonder if you have a 100% latex mattress, as these type of foam really never wears out. I have seen latex foam older than 30 years; gets used everyday and still has the same support factor as when new. Now if you live in a hot dry climate or in the desert, this will dry out a latex and it will start to crumble.

Firm innerspring mattress, not much for sheets over it will be coolest.

Like others said, the more you sink in the less air circulation you have. Unfortunate because it is very possible to pick too firm a mattress.

Have you considered a waterbed? I mention this because of the temperature control features.

Jaywalker Toronado makes some good points, but I still think the gel top is the way to go considering comfort and a cool sleeping surface.

We have just begun selling gel foam mats. Perhaps over the summer I will sneak in a nap on a display and come back with a warmth report.

Gel foam is more inviting than regular memory foam. Our more expensive ones are SOFTER so I sink further in them...