Go buy yourself a new battery . I will only put Bosch S3 or S4 batteries on my own cars . Some of my cars don't move for weeks at a time , but they never fail to start .
Have you had a sparky check the car for anything draining the charge such as an interior light ( especially the one in the boot) , or the radio ?
A flat battery can be boost started as many times as you want without shortening it's life , but it takes around THIRTY minutes of actually driving the car to get it fully charged . Just leaving it idling will take longer , and if you are having to do it every few days it is going to cost you more in fuel than a new battery would cost .
As mentioned above it is not a good idea to leave your car sat running . It can cause allsorts of problems .
Booster packs are a great idea , but you have to pay a great deal of money for a decent one . One I use cost nearly £400 . It is big and heavy and not really suited to be carried around in the boot though .