Just a couple of tips that I haven't seen (if they're already out there, my apologies):
1. Time of day can really make a difference in the appearance of your build, so try all 4 when saving your lot to the Gallery (you can choose to save, view the image that will be generated, and then back out of it without actually saving). If you also post pictures of your lot, consider the same thing. The front of the house might look good in the morning, but the back might look better at another time of day.
For instance, with my Rock N Roll House, here it is in the morning:
http://i916.photobucket.com/albums/ad5/SFsimpics/Sims4/Building/guitarmorning_zps51f79388.jpg
The color is too dark and dull.
In the afternoon:
http://i916.photobucket.com/albums/ad5/SFsimpics/Sims4/Building/guitarafternoon_zpsa6e16a6b.jpg
Better, but I don't like all of the shadows being cast on it.
In the evening:
http://i916.photobucket.com/albums/ad5/SFsimpics/Sims4/Building/guitarevening_zpsd5cefc24.jpg
Look at the way the red color really intensifies, just the effect I wanted! This was the time I eventually chose to save the lot to the Gallery.
At night:
http://i916.photobucket.com/albums/ad5/SFsimpics/Sims4/Building/guitarnight_zpsec3618d0.jpg
I considered it, because I did a lot of work with the lighting, and the house and pool look amazing from the back at night...but that's not what people will see on the Gallery. :p Also, the guitar structure is the star of this build, and the other stuff I did is just icing on the cake. Make sure you don't lose sight of the defining feature(s) of your build!
2. Make sure that what people see in the Gallery is what they will get! I recommend that prior to sharing a build, you always place it yourself from the Gallery in build mode (as if you had downloaded it), and make sure that everything has been preserved. This is especially important if you've used any resized objects. I just saw a beautiful build today, but when I placed it my game, there were no doorways to actually enter the house! I think this was because the builder used an enlarged rock near the door; you could see it in the Gallery shot, but both the rock and the door it was next to were gone upon placement.
