I work as a TA 3 and a half days a week and supply onFriday. I made my decision due to supply being much slower this year compared to previous, I was an assistant to a supply teacher today who was a former principal, and who spent years organising and hiring supply teachers. It was useful to find out from her that there is absolutely nothing to take personally if a school does not ask you back. It is simply the case, I was told that schools are massively busy places and they seldom remember the supply teachers. A good tip she added was to say thank you every day and make a point of meeting the person in charge of supply and basically illustrate how you really love the school and would like to work there. As stated, supply teachers are seldom observed so it is not even on the basis of being thought of as being better than another. However, it is beneficial to raise your profile a bit. This is especially important in these very competitive times for supply. Hope this helps.