DIY: Build a Cheap Variable Neutral Density Filter
If you're not familiar with variable neutral density filters, they're essentially adjustable filters that change the amount of light allowed into your camera lens. Since it's limiting the light to the lens, you can open your aperture for a shallow depth of field or prolong your exposure time for a long motion blur.
These filters can be incredibly cool, but at around $100 a pop, they're not exactly cheap. You can create a similar effect though, with a pair of cheap polarizing filters. It takes a little bit of work, but Peta Pixel has a great tutorial that should have you up and running in no time.
The way it works is pretty simple. Two polarizing filters set up parallel to one another won't block out any additional light. If you turn them at a 90 degree angle, you'll block out a massive amount of light. For anything in between, simply adjust.
This cheaper density filter isn't perfect - you'll still have the polarizing effect on your camera, and you may have a bit of color change too. But for the price, it's hard to complain too much.