Top-tip; If you want to be a better photographer then stand in front of more interesting stuff.
Planning
READ THE BRIEF. You can take the best images in the world but if they do not meet the brief it may simply be rejected from the competition/results.
Set your camera up for the type of shoot BEFORE you leave home. The right lens if you have the luxury of choice, Aperture priority (a good default), White balance, Metering, Max ISO, Vibration controls, Picture delay, Release mode, Autofocus etc. Sure you will change some on location but having a pre-set base is great. (User preset 1 is a great “reset to my style” option if your camera allows that.)
Check the battery (full) and memory card (empty).
Check you have all you need for that shoot; Flash? Different lens? Different filters? Tripod?
If possible avoid mid-day (harsh lighting). Dawn and Dusk + / - an hour are ideal. Yep, set your alarm early!
If possible get to the location at least 30 minutes before needing to take a picture. Potter about to find the best picture. Understand the footing in case it gets dark (it can be pitch black in remote locations).
Useful kit; Tripod, Spare batteries, Headtorch, Sturdy footwear, Warm clothing, Drink/food.
Useful planning; Clear Outside, Local weather, Local tide times, Webcams, Stellarium and Moon phase for night shots.
If doing night shots you HAVE to know your basic camera settings in the dark. Torches spoil night vision for 20 minutes plus.
Composition
Is there a subject; i.e. what is the point of the photo? Is the story clear? Does your eye bounce around - if yes there is no subject = Meh picture.
Are there leading lines, shapes and/or colours taking you to (or away = bad) from the subject? Have you remembered the rule of thirds, leaving room for your subject to move into or look into the spare space?
Should you break the rules for a more powerful composition?
What is distracting from the main image (check edges, check for red)? Check the background for distractions.
Is there depth to the image? For landscapes; Foreground, Midground and Background. There should be content in each area. Consider adding some close elements to frame the picture.
DON’T take every picture at eye level - don’t keep the tripod set for your personal comfort, set it for the image! (i.e. be at the height of your subject). Also, don’t take every photo in landscape, some shots look better in portrait mode. Exercise; on one shoot take one portrait shot for every landscape shot. This forces you to think about composition.
Can you simplify the shot? Think "What can I take out" (by zooming in, or even moving your feet (feel the horror!)).
Check horizon is level - yes you can fix it in post-production BUT you can lose a lot of the wanted image when you do that. Why not get it right?
Check the histogram - is the exposure correct (or a little under)? Zoom in and check sharpness - especially any eyes in the picture. You cannot correct really bad focus in post-production!
Add your favourite tip here:-
Taking the shot
What type of shot are you taking? Usually;
Landscapes need a big depth of field (big F-number),
Portraits, including animals, and birds need a small DoF aka wide open (Small F-number),
Smooth running water, or waves by using a slow shutter speed (0.5-2 seconds ish),
Sports need a fast shutter speed (1/500 sec or faster),
Night shots (small f-stop) also need a high ISO setting (you might need to unlock the ISO).
Consider filters; ND for landscapes and Polarizer for water are good starting points. These help if you run out of ‘room’ when trying the above.
What is the light quality, direction and intensity?
Does an alternative height or angle create a more interesting perspective?
After the shot check the HISTOGRAM and zoom check the FOCUS, yes you can return but who needs that? Far easier to take another shot with the right settings. However don't chimp!
Post-processing
Is horizon level - usually correct if not (along with any geometric building corrections).
Are any subject's eyes sharp? It's ESSENTIAL that eyes are sharp, with catchlights if possible. Discard the image if the eyes are not sharp.
Do you need to crop (for the picture, or for printing requirements) or would it look better rotated 90 or 180 degrees?
Zoom to 100% or more. Where is it sharp? Hopefully in the subject area!
Would a one or two-pixel border help the image? If yes white, black or what?
Thinking about colour, would the picture look better in B&W?
Gloss, Matt, Fine Art, Metallic paper? Scale for your mount size.
Is it good enough to sell? If not it has limited milage for posting/displaying.
Consider returning to the location and doing better having learnt from any mistakes above.
and finally, before you pack up;
Check and charge your batteries
Clean your equipment, especially filters and especially if shooting near the sea
If entering a competition the image title appears to be almost as important as the image content to some judges. Jot down at least three different titles and sleep on them. Our rules; Title {space} - {space} Membership number {space} - {space} PenrynCC.jpg e.g. Short and relevant - 148 - PenrynCC.jpg