I'm guessing your bike is at least 4 years old from your comments, what type of battery is it? the larger one where you can fill the battery yourself (usually white/cream in colour) or the newer smaller type, sealed unit?
The sealed type don't seem to last that well, well not for me and a few others, the old type seem to last longer. Have you tried cleaning the battery terminals up (sandpaper etc) couls just be that and oxidised etc.
If not probably a new battery as you suspect