Witch's Box

It's a good idea to always keep a box of spare bits and bobs by your altar or where you cast your circles just in case something goes wrong.

Element Candles:

Keep spare candles to represent the elements you cast the circle with, whether it's fire, earth, water, air, metal, wood, or spirit. It's always good to have some spare.

Spell Candles:

Keep at least one spare candle of each color. Have a couple extra white ones because you can infuse these with runes or tie a colored ribbon around them to show the color you're using.

Strings/Ribbons:

String and ribbons can be used in spells to create chords or to symbolically express connection or release. They can also double up to decorate candles.

Spoon, Knife, Tweezers:

It seems odd, but this is a good idea: a spoon to help mix potions, a knife if you want to carve into candles, and tweezers in case you drop something small or your wick is trapped by candle wax and you want to get it out without making too much mess.

Mirror:

Mirrors are used in quite a few spells and it's always good to have one spare.

Heat Proof Dishes:

For candles and burning things.

Matches:

Of course, you can substitute this with a lighter, but I prefer matches.

Small First Aid Kit: 

It has nothing to do with spell casting, but in case you get burnt or cut whilst you're casting a spell you want to be able to sort yourself out quickly. If you don't want too much fuss, just have a box of plasters, antiseptic wipes, and an instant cold pack.

Bottles and Vial Glasses:

In case you break something you're about to fill with a potion or tincture, make sure you have some spare.

Smudge Stick:

It's always good to have a spare smudge stick.

Salt

Salt is good for everything. Everybody needs salt.

There are many other things you may want to add to your own box, but I feel these are most important.