What should I start doing to improve my prospects since deciding on an academic career? So, I have had a year out to work and am now going to do my Master's degree in Computing. I have decided to fully embark on a career in academia.

What can I start doing NOW that will help improve my academic reputation, credentials, CV and career prospects? For example, should I do more writing, set up a technical blog, find research opportunities, what projects should I start, organisations to join etc.?
Try to find active researchers who are willing to let you do a research project with them (easier said than done, but if you're offering to work for free in your spare time without huge amounts of hand-holding then people will sometimes give you a chance), and then collaborate with them to write a paper. Papers, particularly high-quality papers in good venues, are the primary currency in academia. Writing a technical blog might be fun, but realistically won't affect your academic reputation one way or the other (it's unlikely that many academics will see or read it). Similarly, joining organisations isn't on its own going to have much impact on your academic reputation - people generally don't care if you're a member of e.g. IEEE, they just care if you've done any useful work lately. Helping organise seminars that are popular in the field (if you can find a way to get involved in that) might get you noticed to some extent, but only counts a bit (people hiring researchers don't want to know whether you can organise talks, they want to know if you can produce interesting research).

Basically, the main thing that matters is (good) papers - try to find ways to write them, preferably with other people, but on your own if necessary (the latter is hard and less effective, so try to avoid it if you can). Try not to waste too much time on things that consume your energy but for which you don't get any reward. Enjoying them can count as a reward, incidentally - by all means do fun things just because you like them, just don't expect them to have a huge impact on your reputation.

Source: It's Sunday, and I'm writing a paper, so at the very least I believe the advice I'm giving :)