Anne, I have the Extech 100. This meter features a flat tip which can be used either in liquids or on a solid surface such as a cheese.
I calibrate only occasionally, generally if it seems that my results are not quite coming out the way I would expect - maybe every other month or so. Generally I find that it is only out by .1 at the most when I re-calibrate.
In between calibrations, I wash the probe with a tiny bit of dish detergent and water, and dry it with a paper towel. Then I take my reading - and take more than one reading , since sometimes the first reading will be off compared to the others (perhaps a little bit of water residue). I then again wash and dry the probe, and then put it back in the storage solution (which in the case of the Extech, is simply some 4.0 buffer solution). I repeat this wash and dry routine for each reading - probably a bit excessive, but it helps to ensure that a film of fat is not interfering with the reading.
I hope this helps!