Mark gave a great answer, of course suitable for concentration is a subjective thing so remember my answer is based on what works for me when I'm coding at work, or concentrating on a problem.
I would add that there's a distinction between relaxing music and music that helps you concentrate, and there's a certain crossover between the two.
There's a balance between being predictable/boring and over developing your piece in general, but in the case of writing study music I'd say you want to be as close to the predictable end of the spectrum as possible without becoming boring so that the listener's active brain isn't distracted by the music. If the listener is actively paying attention to the music it might even sound boring, but you're not aiming for the active listener in this case, you're aiming for the passive listener.
There was a song called weightless which was created between scientists and musicians that is worth looking into, being designed to relax and is able to slow the heart rate over the course of 8 minutes. An important factor to consider is how long it takes for the human brain to become familiar with a given soundscape, because that time will indicate when you need to start shifting away from it.
Natural noise like a crackling fireplace, pouring water or natural sounds seem to have a neutralising effect, you can explore all sorts at http://www.freesound.org/
tempo wise, based on the assertion that you should be close to boring I think a little above heartbeat speed might work.
I'm tempted to say keep it in 3/4 or 4/4 to keep the ear from noticing anything unusual with the music, but the occasional triplet would be alright.
Nothing piercing, or overly strange to the audience you're catering for. (Bagpipes might work if you're used to the sound, but to most people their sound will draw conscious attention to it) it is good to have only one or 2 main sounds to focus on at a time. If there's no leading sound/instrument for a long time you again may get too close to the boring territory.
- Key changes might be one very good way for you to vary the music over a long period of time, but use them sparingly!
- Use dissonance very sparingly also
- Take any necessary changes in the music gradually
Some Research music that might be worth exploring
- Slowed down songs on Youtube
- LongPlayer - a computer generated piece that lasts 1000 years
- Vangelis - China
- Faithless - Sunday 8pm
- Younger Brother - The Last Days of Gravity
- Bach - the Well tempered clavier
- Pink Floyd - Shine on you crazy diamond
Hope that helps!