There are a range of products available that can be used and manipulated at little cost to the rider. One of these is the humble "snap-band" - these are usually worn around the wrist or ankle, but they can be used in other ways.
I personally found them to be good on the bike itself. On frames where you would not wish to use 3M Scotchlite reflective tape (for reasons of adhesives making a mess, or temporary installment) they can be installed with cable ties. Make sure to angle the end of the tie away from the legs of where it may snag on clothing, body or part of bike and it'll be fine.
|Seat tube, cable tie angled in towards rear wheel|
Spoke reflectors are great too. They only cost a few quid for a box of about 20-30 and just snap on the spoke. As the wheel turns in front of normal vehicle illumination it conveys movement and a sense of "bike". Again, they are a great temporary measure that can be removed should it need to be.
The spoke reflector can also be made secure with a little strand of tape (perhaps 3M scotchlite, even). I've had a few ping off when washing the bike down at the weekends and it stops this happening.
The great thing about these tools is how even a dark coloured bike can be made to be brighter in a matter of minutes. The other great thing is that these reflectors are curved and reflect light from a variety of angles.
Obvious caveat has to be said - these measures are no replacement for safe and legal riding and depend on other road users direction of light being in the right place at the right time.