I'm losing more than I win on Live Chess 15:10. What's the best way to improve?

I find that playing a slower time control is better for actually learning how to improve your game.  Maybe play some turn-based games for awhile and try to really understand what's going on before making each move.  Then your good habits should translate to better results in the faster time control.

Just by glancing through a couple of your recent loses in standard time control, you overlook simple forks and place pieces on squares which are not safe i.e. losing the exchange with no compensation. The answer to your question is simple, always look for all possible Checks, Captures and Threats. 

Thanks CasualJoe, I'll try and get a regular game of turn-based chess going. Any more suggestions? 

You're right cuneglas, but how do I get better at  looking for all possible Checks, Captures and Threats? Given the time contraint, I panic & guess. Are there any guidelines on optimal checking for Checks, Captures and Threats?

One of the things that helped me the most is this -- once I've decided on my move, take a little time (maybe just 5 or 10 seconds) and try to find your opponent's best response.  It's amazing how many times I discover that my intended move was a total blunder.

Board vision can be improved by studying many simple tactical problems i.e. the pin, knight forks, removal of the guard etc. Note the word simple. More complex tactical problems serve to improve calculation, more than board vision in my opinion. Of course you can work on more difficult problems as you improve. Try the free 'Lucas Chess' program, which is an excellent training resource which has many tactical positions organized by theme. 

The last game I played is typical, apart from me winning :) I think I followed basic opening strategy well (?) - develop pieces early, central, advance on the King if you can... so I had Knight on g4 protected by B down a long diagonal. My opponent advanced a pawn and cut off the long diagonal and I just forgot their Q was attacking along another long diagonal! The Kn was just swept away for nothing... There was an obvious retreat for the Kn as well, obvious in hindsight and with plenty of time. Your excellent idea wouldn't work here CJ, 'cause the Kn was just sitting there, forgotten...

The tactical training sounds like a good idea cuneglas  - I'll try 'Lucas Chess'. I have two beginners chess books to hand, "Everything Chess" by Kurzdorfer and "The Mammoth Book". Would hitting those hard help, do you think?

Maybe it's age & rust, I used to do well against my friends at school, many years  ago, but haven't played much since...

really.. using chess mentor is one of the best ways to improve your game... :)