Acrylic - Kent Marine Scraper with the Plastic Blade.
Glass - I used a razor blade, but if you don't want to put your arm in the tank the Kent plastic blade scraper works just fine.
For Corners - I use a long handled brush. Not sure the brand, it has a blue handle and white pad.
Glass is very difficult to scratch, acrylic very easy. Keep your ears open while you're doing it so you can hear it if you mess up. Never scratched a tank from the brush/blade; sand grains getting caught should be your main concern.
I worked for an aquarium service company for a while, so I got pretty adept at cleaning the tanks in record time. You can go to town down at the gravel line with the plastic blade and usually not scratch anything up. DO NOT use a sponge down near the sand.