During the final fight with Mettaton EX, if you have certain fashionable items, such as the Stained Apron or the Cloudy Eyeglasses, your ratings by default are boosted. Furthermore, certain items being used, such as the Glamburger, will boost ratings. This will make it easier to end the fight sooner.
Developers' Room
In Snowdin, there is a large door that normally doesn't open for you, To open it, complete the credits without touching a single name, and beyond you'll find the developers' room.
Easily Defeat Muffet
To defeat Muffet in a single turn, you can use one of the Spider Bakery items - the Spider Donut and Spider Cider - you can buy significantly earlier in the game. Use one during the fight and it'll end immediately.
Hard Mode
To activate hard mode, enter "Frisk" as your starting name. You'll get a warning that this will "make your life hell"; confirm to begin playing hard mode. The changes include not being able to flee on the first turn of battles, toughened and new enemies, and only being able to hold three pieces of candy, among other things.
Weaken Asgore
If you save the pie given to you by Toriel from the beginning of the game, and use it during the final battle versus Asgore, you will weaken his attacks noticeably; being relatively hard to dodge, this makes the battle significantly easier.
Boost Mettaton EX's Ratings...