I'm trying to turn a vibration motor (3v) on and off rhythmically using my arduino. I'm trying to use the 2N3904 and I have the pin connected to the base, motor - to collector and the emitter to the arduino ground and battery ground. I've tried putting a few different resistors in between the pin and the base, but I haven't been able to get anything running yet. Does this set up sound correct? How do I know if the resistor I'm using is too big or small?
I don't have much of an electronics background at all, so I apologize if this is incredibly obvious. Thanks for any and all help! 
First we need to know how much current the motor draws when running at 3v. Why don't you hook a DVM meter in series with the motor and it's power source and measure what the current draw is. Then we will know the collector current required for the transistor and that determines the base current (X gain) which then allows for proper sizing of the series base resistor. You may find that you require a beefier transistor, but it's all a guess until we know the motor load current.
Lefty
As was suggested you should determine the current draw of the motor.
The pager motors that I have used were around 75mA. I used a 3904 with a 4.7K resistor in series with base.
You can tell your resistor is to high by measuring the voltage between the collector
and emitter when the motor is on. If the transistor is completely on (saturated) you should measure a few tenths of a volt. If not then you need to increase the base current
by reducing the series resistance.
You can calculate the resistance by looking at the Vce(sat) specification in
the 3904 datasheet. You need a resistance sufficient to generate a high enough
base current to saturate the transistor.
(* jcl *)