Other than cleaning your shoes on a regular basis, these tips will help you maintain that "new" look to your shoes.
1) Store your shoes in a cool, dry place. Keeping them out of sunlight and dust will help you maintain the shoes looking good for when you're ready to wear them. There's nothing worse than opening a box of shoes you had been storing for a long time only to find out they've yellowed due to the fact that they've been dried out and stored next to the heater!
2)I know some people like to display their shoes like trophies, but this can also be a bad thing. Direct and constant light tends to dry out the shoes causing materials to break down and yellow.
3) Silica packets help by taking moisture out of the air. Moisture will eventually ruin your shoes. It's recommended that a couple small ones be placed in the box. eBay is a really good source to buy silica packets. Another suggestion is hit up your local electronics store. Ask them and their delivery guys to give you the little packs that fall out of boxes. Tell them to just put a tiny box somewhere and toss them in, and give them $5 at the end of the week or when the box is full.