Lighting your drops properly is essential.. but it can be expensive. A single decent (new) flash can cost upwards of £200.. I'm on a budget so this is a guide to how to get good results for minimal £££$$$
Speedlight flashes work by powering up the flash to a point, and then they power down again. Bigger the flash the longer the flash. So if you want to freeze the motion in your drops and get those 'caught in the moment' shots without blur.. you need a quick / low power flash of light.
This means you have less light to work with and will affect what Aperature and ISO you have to use. It's about now you have to start trading off something and it's Depth of Field vs Speed of Capture vs Noise (using high ISO)
You *can* take drop pictures using the on-camera flash and get some pretty awesome effects. Milk drops seem to show movement less than water so if your using on-camera flash you might want to think about those..
As you can see on this the droplets above the crown are streaking and not frozen by the on-camera flash.
This technique uses a long exposure in a darkened room (1.5 seconds or more) and a flash seperately fired to freeze the motion.
Flashes I have used
Vivitar 255 - Not found very often on ebay but CHEAP.. I got mine for £0.99 + postage. Can be dialed right down to freeze the motion very well but it's not very bright, have issues with lighting.
283 you can get from ebay for £25
285 you can get for £45-50
The beauty of these flashes is that the auto-thrystor on the front is a plug-in. If you can get the vari-power module for this flash you can turn it down to 1/16th power on the flash and 1/16th power on the vari-power module and freeze the motion VERY well.
Alternatively you can create your own vari-power module using a variable resistor (see http://www.hiviz.com/ and various other sites try googling vivitar 283 hack)
Speedlite flashes of all kinds can be used to freeze the motion however, I think 1/64th power is around the sweet spot for this. The strobist blog (linked above) has a quite a lot of information in it.
Same kind of splash using a flash set to minimum power handheld..
Next.. Making drops!