Photoshop tips and bits

I thought I had already posted this thread but I can’t find it anywhere so:

Considering how many photoshopped Typhoons and Rooivalks have been passed off as new Chinese developments (don’t get me wrong, the Chinese are certainly not the only guilty party) on this forum and on the web as a whole I thought of a few questions.

Now, I’m no expert with photoshop I must admit but still I can’t help being impressed with images where the ‘artist’ was able to change the camo pattern, the roundels and maybe the background as well and still managed to make it all look (almost) believable. Frankly its the camo pattern change/swap thats got me stumped. I’ve tried to alter the camo pattern on a couple of pics (which I won’t post due to embarrasement) but I can’t acheive the kind of results I’ve seen time and time again. My images look very obviously fake and amature.

Does anyone know how to change the camo pattern on an image of an aircraft and to make it look believable? I’d love some tips from those better versed in photoshop techniques than myself.

Thanks in advance for any useful replies!