I'm still waiting for my Adafruit Pro Trinket to arrive (it takes a while to get them here in Greece), so I used an Arduino Leonardo instead.
I've used two accelerometers so far. I started with the ADXL377, which I could find here i Patras, Greece, but it turned out to be completely the wrong tool for the job. It measures acceleration of up to 200g, so its resolution is really bad.
I've ordered an ADXL337, which is similar to the 377 but measures up to 3g and has much higher accuracy, and I'm waiting for it to arrive.
In the meantime, I managed to borrow an ADXL345 from a friend, and I may very well end up using this, because it supports sleep mode, interval and threshold based interrupts and I2C/SPI communication, which will probably be necessary to lower the power consumption to a paint where it can last a few days.
In any case, I've managed to gather some data of my acceleration pattern while sitting, and I've posted them on a GitHub Page.