I do not think there is anything "better" to do - go out and find inspiration there or plan ideas beforehand. It all depends on each individual and what s/he tries to accomplish. A street photographer better look and find it on the spot. I on the other hand, do a lot of prepared shots for my portraits. Usually, I find an idea, asks someone if they'd be interested, and then we choose a place, clothes, make up necessary for the idea to pass in the image. I used not to, to train just getting nice pictures technically, or nice expressions, but now I feel I'm at a place where I need to focus more on ideas I want to convey.
Where do I find my ideas? Mostly in what I love, or things that touch me, anger me, etc. I love role playing and steampunk look, so I decided to create a project around that. Now for a while I have been thinking about discrimination, so I'lve started to shoot a bit on that too. I usually just think and jolt ideas down in a notebook, first on what I want to say, then on visual means to represent it, and I let it grow until I have a better image in my head before doing any actual shooting. I want to work more on that, I feel like there is to much left to randomness in my pictures still.
I think all the images I see inspire me, but in a more inconscious way. I rarely think, "I love this picture, let's try to do the same" (through I think it's a good exercice, and there is nothing wrong with it as long as we are honest and link to the person's work if we put it online, s/he found the idea and should be credited for that). It's more, "Oh, I love the use of light here, I'm guessing it's so beautiful because they did X, I should try this angle/technique in a portrait". I add it to the list of things I need to work on / try out.
As for new ideas - well. Obviously there are subjects that are more often photographed than others (especially in nature). I think it's okay - we should just photograph what we love, and if it's something a lot of people love to photograph too, there is nothing wrong with that. I do love to see new concepts, because it makes me think and react more. Something I have seen a thousand times, even if the picture is good, will often bore me. There was a point where I just couldn't look at waterdrops and ladybugs anymore, I just felt like they were everywhere. But I think even the same idea will not be conveyed in the same way by different artists, based on their sensibility. I don't especially try to be original or not, I just try to do what I love.