First, you really do need a screenshot capture utility -- having to drop out of FS and paste the contents of the clipboard into Paint or whatever isn't really going to encourage anyone to take a lot of screenshots.
There are a lot of choices out there. Googling 'screenshot capture' is a good start to find one that works for you. 'SnagIt' is a full featured tool you can use for free, or pay for if you want the extra features. However you might want something very simple, which doesn't take up much space when the sim is running. I use 'Grabber2K' which is fairly streamlined, but not supported any more. However googling 'grabber2k' gives a lot of sources.
(You'll notice that I'm pushing Google a lot, rather than supplying direct links. There's a reason for this, but that's another story.)
Remember, though, that FSX has a built-in screen capture tool -- just press 'V' to take a screenshot.
Once you have set up your screen capture tool to your liking, just start it up before you fly and it'll be ready to go. Many use the Print Screen button to make the capture, which is handy because it doesn't interfere with any FS keys. If you can set yours up that way, it makes sense.
Now if you suddenly see something special on your screen, you can just press the Print Screen button and your screenshot will be saved as a normal image file.
However, there are a few things you can look at before you press that button. You may wish to pause the sim, and look at the shot from different angles. The lighting may be better from another angle, or you might be able to include a bit more -- or less -- of the ground, or get a nice cloud in the background.
You can even change the time of day while you are paused, to try out different lighting. Most good photographers appreciate the nicer light available early morning and late afternoon, rather than the stark mid-day sun.
You can also change the graphic settings if you wish. Many of us fly with fairly streamlines settings, to help performance, but we do have the ability to turn up the settings just for the screenshot. FSX makes this easier, being able to save different settings, but FS2004 works just as well provided you remember to put your old settings back. For instance, you might want to turn up the mesh or autogen settings to give a bit more ooomph to the shot, or turn up the cloud draw distance to really bring the sunset details out. One thing I'll ask for anyone taking screenshots of any of my scenery -- no matter what your normal settings, please consider turning scenery shadows on to take your screenshot! Shadows are a very important part of the world, and here in NZ we have some really impressive shadows -- a world without shadows, is a very dull world indeed
Why not change the weather? Maybe a stormy theme would add a little more atmosphere.
My favourite -- go into slew mode to rotate your aircraft if it'll give a better angle. Or move it a little higher/lower.
No doubt some of the folk who have won screenshot comps here can supply a whole lot more tips, but mainly it's just fooling around, and practising.
