While there are several preservatives available now, the most important thing is that the preservative used works. When I made lotion the first few times, I just followed a basic recipe and didn't give much thought to inhospitable conditions for the preservative. Looking into them more now, there are various factors to consider. I've listed a few of them below.
This one is an easy one to adjust for. Find out the temperature at which it's safe to add, and then during cool down, check the temperature and add the preservative when it's low enough. A few can withstand higher temperatures, so you might be able to add those to the water phase (possibly oil phase, though I don't recall seeing one that's added to the oil phase).
This is one of my favorite considerations. You get to play with pH test strips or a fancy pH tester, though for now I'm sticking with the test strips. Preservatives seem to prefer acidic environments, though some will work at higher pH levels.
3. Reactions & Compatibilities
You need to make sure your preservative won't react with another ingredient, leading to something like benzene in your lotion. You need to make sure it works with all ingredients, such as cationic/non-ionic/anionic surfactants and your emulsifier.
4. Light exposure
I've only seen this so far in reference to Tinosan, but if you use that preservative, you should make sure to avoid exposing the preservative to light, and you will need to test the final product if you use anything other than opaque bottles. Tinosan actually sounds fairly interesting since it contains silver citrate. I think that sounds pretty cool.
Some of the natural preservatives offer benefits on top of preserving your product. For example, willow bark extract is a natural source of salicylic acid-like ingredients, so it can improve your skin.
6. Broad Spectrum
If the preservative used kills only one type of nasty (bacteria (gram positive and negative), yeast, fungus, or mold), you'll need to add another to kill the others.
Usually you add preservatives to products that contain water, but you might add one to an oil-based scrub that will be exposed to water. In this case you need to find a preservative that is soluble in anhydrous solutions.