Well, it's a way of thinking. A way of viewing the world. A way of questioning. You need to look at the world with a childlike curiosity, all senses 'on' and both your eyes and your mind wide open. A child wanders about and tries everything out, plays with things, experiments, explores, questions and attempts to figure out the world around them. You need to pretend you're a curious little ankle biter. Once you're in the childlike 'open state' you absolutely must learn to ask key questions. The simple 'Why is it like this?', 'What if it did that' type questions really get you pondering.
Too often in our day to day life we are awake physically but asleep mentally. We aren't fully alert or attuned to what we are doing and how we are doing it. As a result of us simply not being in-tune with our surroundings and interactions, we don't notice things hidden in plain sight and we get no ideas. Life just happens around us, we think our surroundings are the way they are because 'that's just the way they are'. We don't for a second think that we ourselves can make change somehow. To remedy the status quo and the 'I don't have ideas state' you've got to learn to see, learn to be curious, learn to question things, learn that everything is changeable. Once you learn the above then ideas start to 'waterfall' and 'flow'.
The critical things you need to do in the ideas game, is to constantly observe the way yourself and other folks interact and behave with things. For example, one day I was sitting in my room in Sweden's natural deep freezer (Umeå) and I noticed that whenever I unplugged my laptop the charger cable fell onto the floor. I didn't just think of it as an unavoidable annoyance. I asked myself the question 'Why can't I make something that keeps the cable on the desk?' I decided to act on the problem I faced and create something to solve it. The cable fondler was born.
Good observations result in good ideas. In 1941 a chap called George de Mestral went on a hunting trip with his dog in the Alps. He returned home, sat by the fireplace and was plucking out some burdock burrs (seeds) that had attached themselves to his clothes and his dog's fur. He then got curious and decided to have a look at them under a microscope. He saw a bunch of "hooks" that caught on anything with a loop, such as clothing, animal fur, or hair. Velcro was born. What a 'sticky' idea ;) James Dyson was vacuuming when realised that his vacuum cleaner lost suction as the bag filled up and his vacuum totally sucked as a product. He didn't just sit there and think 'oh what a shame, these vacuum cleaners don't actually work very well'. He took a stand and said, "this vacuum cleaner is rubbish, I will make a better one", and he did! The Dyson was the worlds first 'bag-less' vacuum cleaner and sucks a golfball through 8ft of garden hose, almost.
You can also look to other countries and other industries and borrow/improve/combine on some ideas you might see there. Sushi was once only served in Japan. Groupon was one of the first discount sites, but since them there were a bunch of others in almost ever country from Australia to Estonia.
“Ideas are like rabbits. You get a couple and learn how to handle them, and pretty soon you have a dozen.” – John Steinbeck
Once you have found an idea in the form of a problem to solve or an improvement to something, keep going. An idea is just an idea, a vague combination of thoughts, until you move to the next step and start making it real. Don't just sit there and talk about it, get real with it. Ideas don't just pop from your head as readymade products, you have to channel them out and create them in the real world. Get making! If it's a physical thing, grab the cardboard, sticky tape and x-acto knife. If it's a web application, get sketching, get illustrating. Get the idea as real as you can. The more tangible your idea becomes the more you start to notice ways you can improve it and the more you progress.
During the whole idea generation process share your ideas. Share them with your friends, family or whoever, kick the idea around as much as possible. Ideas starved of daylight, action and people, die. Share your ideas freely, an idea in the minds of many makes magic.
Progress procreates progress. Once you have an idea and are working on it there will always be times when you struggle to gain motivation. Keep buggering on as Churchill said. Spend some time each day working on it. Kept at it, see it through. James Dyson was almost broke when he came upon the 5127th prototype of his first vacuum cleaner. Actually, he is now 'Sir James Dyson', he kept at it and one day the Queen tapped him on the shoulder with a sword. The more you get into it the more your progress inspires you to keep going. Ideas pay off when they are seen through.
One last thing I have to add here, is that you won't simply find any ideas just sitting still, watching 'reality' TV. George de Mestral didn't invent velcro whilst sitting, he was out walking in the Alps, Sir James Dyson was vacuuming. You need to keep moving, participate in activities, build things, travel, meet new people, surf the interweb, read, watch, dream, discover. A mind starved of inputs remains stagnant and sterile. So go on, get out there, grab your gear, open your mind, suck in some fresh oxygen and enjoy the new world around you. Observe this world with fresh eyes, question, think, get an idea and start creating something useful!