Concentration is a funny one for me. At times, like today I can be so completely absorbed in what I'm doing that I fail to notice practically anything else around me (hyperfocus). This usually occurs when I'm doing something that interests me and occassionally happens when I'm doing interesting Uni work that I know has to be done soon, like assignments. I can just sit at the computer typing it up for literally hours at a time with few breaks. I wouldn't reccomend that though, it's very tiring and gives me a headache!
At the other extreme, I can find it extremely hard to stay concentrating. Recently I've been trying to do revision. I find it hard to set myself a timelimit, say half an hour because I'll spend the first twenty minutes daydreaming!
As for tips, everybody says don't do too much at once but I seem to work in intense bursts of concentratino that last a few hours sometimes and then fade away as quickly as they come. I take advantage of that. Like today, I've actually been quite focused
However I do agree with taking regular breaks and if you just can't concentrate it's better just to stop than to spend ten minutes reading the same sentance. It's helpful to get other people to test you sometimes - they can help you stay focused.