Good question. There are as many answers, though, as there are photos to combine. It all depends on what's going on in the two images. There's no magic "make these match" button. It's a question of artistic judgment, far more than of tool knowledge.
It starts with taking a good look at both images, and noting the differences and similarities. Do they have similar coloring? Is the lighting on both coming from the same direction? How intense is the light in each? How similar is their photographic distortion, meaning is one more fisheyed or more orthogonal than the other, due to differences in camera lenses? Does perspective match between the two? Are they both similarly focused? Do they have the same kind of grain to them? And that's just for starters. There are a million variables.
Here are some basic tools you should get familiar with, to get started (assuming Photoshop):
You'll find the Match Color tool (Image -> Adjustments -> Match Color) to be very effective at making two differently colored photos look similarly hued. It doesn't always work perfectly, but usually it will provide you with a good start. Here's a pretty good tutorial on it:
http://www.photoshopessentials.com/photo-editing/match-color/For evening out lighting, you'll find the High Pass filter to be very effective. (Filter -> Other -> High Pass)
For fixing perspective problems, the vanishing point filter can work wonders. (Photoshop Extended only: Filter -> Vanishing Point)
Layer blending options can be great. A drop shadow here, a bevel there, and things can jump from "lick & stick", as you put it, to plausible co-existence.
Adjustment layers are essential. Spend some time learning about their various types, and what they all do.
Don't forget about good old fashioned hand painting. There's often no substitute for it.
Those are a few things to get you started. I'm sure others will recommend more. And I'll take this opportunity, once again, to plug Luke Ahern's book, "3D Game Textures: Create Professional Game Art Using Photoshop". It covers just about everything I mentioned, and a whole lot more. It's a must-read for any budding texture artist.