It's always a pretty good idea to pay close attention to EVERYTHING you can see in your viewfinder, like the surroundings and backgrounds, and not just your main subject. Just of the (many!) aspects to watch out for when you are framing your photos is the horizon - which can actually mean ANY horizontal lines in your image, so don't think you've got away with it if you don't shoot landscapes :)
Let's get going with the 5 "Rules" for the horizontal lines in your image!
#1 - Don’t Let the Horizon Cut Through The Head
If you have a horizon line in the frame, try to check the background when you are taking the picture to make sure the horizon line is not going through the head or the neck - this usually means getting getting yourself lower or higher than your subject. I am constantly guilty of breaking this one! There's no fix for this one in processing, so get it right in camera :)
Here's an example of an image that has a horizontal line running through the neck - it's not the end of the world if you get this, but try not to do it! This image also has another no-no regarding horizontal lines......
#2 - Get Your Horizons Straight
If you have a horizon in your picture you should also aim to get it straight! Having wonky tilted lines in your images causes your viewers to feel a little unbalanced, so unless you are deliberately trying to make your viewer feel unstable, get your lines straight! You can do this in camera - one way to do this is to use the rule of thirds overlay that most cameras have and line up the horizon with that, although you might even have a dedicated "spirit level" in your camera too! (You can get also get actual spirit levels for cameras but I can’t help feel that is taking it too far!) If you are shooting an activity and don't have your camera on a tripod, it is likely that you are not going to get this spot on in camera - in which case you can fix this very easily in processing. Just leave a little bit of extra room when shooting to allow for the fact you will lose a little of the image when you crop it.
#3 - Don’t Place Your Horizon in the Middle (Generally Speaking!)
This is one of those "rules" that isn't really a rule since you can break it to great affect. Generally speaking though, It looks more visually appealing to have your horizon run along one of the lines in the rule of thirds overlay rather than the centre of the frame. Using the high line for your horizon creates depth, and using the low one gets rid of any boring foregrounds. Or if you have a bottom and top half that are equally important, break the rule and have the horizon in the middle. (See, not really a rule at all!)
#4 - This applies to all straight lines!
Please also note that these tips apply to other ”horizons” in your image - say, a table top or a hedge, or window lines (like in the image below) and so on. Any line going across your image should follow these rules!
#5 - Watch For Distortion Due to To Your Lens
Finally, your lens may distort the horizon lines in your images - wide angles can be pretty bad at bending the lines and making straight lines look curved. Not a damn thing you can do about that in camera, but in processing, it's usually as easy as one click of your mouse to get rid of. In Lightroom or ACR, go to Lens Corrections in the Develop Module, and then click the little box that says Enable Profile Corrections. This usually sorts it out for you, but if not, you can tinker with them manually (but beware, that way madness lies)
Actually, whilst we're in this box - see that little button that says "level"? That will try to level out the horizons for you too :)