One problem I regularly see with rove beetles, particularly with some of the smaller groups, is that the abdomen shrinks as it dries, telescoping inward and potentially obscuring important characters. A fairly easy way to avoid this is by soaking the beetle(s) in acetone overnight, though I think 3 or 4 hours would be sufficient. The beetles are removed the next day and placed under a heat source (I use an incandescent light bulb) for a few minutes. The acetone tends to harden tissues and as it rapidly evaporates in the heat, the soft tissues remain rigid. Thus you get a well-preserved, un-shrunken specimen, although results do vary somewhat. One downside to this is specimens that have become distended from preservation in fluids will retain this appearance, but I don’t think this is a huge issue.
I use a similar technique for small flies and wasps.