You could try experimenting with some subtle EQing.
For example, try adding some 'warmth' with a little gain around the 150-300Hz range. Be careful though - too much will make things sound muddy and/or woolly.
Also try cutting a little around the 3000-5000Hz range. This will take some of the 'edge' off the sound, although again be careful, as this is where a sound's 'presence' often lives and if you cut too much you'll lose it in the mix.
Most importantly though, these are very broad brush rules of thumb and different instruments will react differently to these changes, so they are not a fix-all solution. Other sounds present in the mix can also make a surprising amount of difference, so try to tweak with the rest of the mix playing at the same time if you can.
Make small, incremental changes and listen carefully to the results as you go.