In reply to colina: If the voltage at the battery terminals isn't going up when the engine is running then there is definitely a problem with the charging circuit: either the alternator itself, or the regulator/rectifier.
However, if you've let the battery go completely flat a number of times then there's a risk that that could have become damaged as well. I always keep my bike plugged in to a battery tender when it's in the garage. You need to be careful which one to use, though. I lost two batteries to an Optimate trickle charger before I dumped it and got an Acumen Platinum smart charger instead. That works like a charm, and does seem to be really "smart", unlike its predecessor. I've been on the same battery ever since I started using the Acumen.
To hook the charger up to the battery I use a 12v socket on a fly lead. I use one designed for marine applications which has a rubber bung to seal the socket when it's not in use. I route the wiring out under the saddle. It has the advantage that I can also run 12v accessories off it if I want to, such as satnav, or heated gloves/waistcoat. You need to make sure that you get the polarity of the socket and the plug on the charger the right way round - or the "wrong" way round in the case of the charger plug, otherwise you either won't charge the battery, or your accessories won't work.
I also have a small digital voltmeter on the inner fairing so that I can keep an eye on the charging voltage when I'm on the bike. That way I can see if the regulator/rectifier is on the way out (a common problem with the model of bike I have, but I've been lucky so far, touch wood). It would probably also let me see if any accessories I did have plugged in were causing too much of a current drain.