When you go to bed, TORIEL gives you a slice of Butterscotch-Cinnamon Pie which heals All HP.
After you get past TORIEL, put the pie in the box and leave it in their until you meet ASGORE, the final boss.

Try to get and save as much money as possible during your adventure.
Simple enough, right?

When you reach Alphys' lab, she has a fridge with a packet of Instant Noodles which you can grab.
MAKE. SURE. YOU. GRAB. THEM!!! (And put them in the box until you meet ASGORE.)

During your adventure, you'll find Sans selling... "Hot dogs".
Make sure to buy one and keep it 'till you get to the last room in the MTT Hotel.
Give it and a white pup will take it and give you a Hush Puppy which heals for 65 HP.

KEEP IT!!!

The CORE monsters give the most amount of consistent GOLD.
Grinding here for the large sum of GOLD is worth it for getting some of the best healing items!

Once you get a good amount of GOLD, go back to MTT Hotel and buy the Face Steak (you can only buy one) and a couple Legendary Heros.
After all that's said n' done, your inventory should look a bit like this.
(Ignore the snowman piece, it's not important NOR will it be eaten.)

Ah, the weapons n' armors.
So many to chose from.
From a Toy Knife to an Empty Gun.
From the Manly Bandanna to the Cloudy Glasses.
Honestly, just use whatever you want, dude.
You'll eventually overcome ASGORE!