Mixing washes, filters and other settling liquids
If you're familiar with the AK-Interactive and Mig type of washes, you'll know that they're enamel based, and over time settle quite persistently to the bottom of the bottle. It's difficult to agitate the bottle enough to dislodge the "skin" at the bottom just using your trusty arm, and it's a pain to have to get a stirrer involved, which invariably ends up wasting some of the goodness at the bottom. Using a paint brush has the same problem, and adds the probability of ruining your paintbrush into the bargain.
A simple solution is to add a ball bearing into the bottle. I picked up a pack of 50 x 6mm chromed balls for a couple of quid on eBay, and popped one in each of my many bottles of washes, filters and paint effects. A quick shake, and that persistent scum on the bottom of the bottles is gone. Just remember to be careful not to agitate the bottles excessively, as even plastic bottles could crack under impact of the ball bearings at sufficient speed.
Why did I choose 6mm? Mainly because I knew from my Airsoft days how big they are. They're also a good weight whilst not too heavy, and are large enough not to disappear in the bottom of the bottles. I chose chromed balls to avoid any contamination of the mixture due to leaching of any impurities from the bare metal.
Hopefully that one should keep your washes fresher for longer
Mike.