Some people have asked recently about the best way to break in a new glove. I would recommend more than anything that you try to use little or no oil or glove conditioner on the glove, simply break the glove in by playing catch or using a glove mallet. Find a hard throwing partner and play catch a lot. When you sit on the couch, pound the glove mallet or your fist into the glove. Take the glove to the park and have someone feed the pitching machine at a higher speed while you catch the balls it throws.
Never leave your glove to sit without putting a ball in the pocket. I like the softball sized wiffle balls myself. I keep one in every glove we own, and I wrap the glove around it using a velcro strap used for securing hockey shin guards. It works like a charm.
If the glove gets dirty, it is okay to use a little conditioner to clean it. Personally, I like the Nokona brand glove treatment that is available at Dick's Sporting Goods. I am told it is simply petroleum jelly, so that can be used as well. Another good conditioner is gel shaving creme with lanolin. The lanolin softens the glove and also kills microbial nasties on and in the glove. Whatever you decide to use (Nokona, petroleum jelly, or the lanoline shave gel), use it sparingly. Too much oil will weigh down the glove. Put one tiny drop in the middle of the palm and work it in hard. Don't smother it in goop. When finished, wrap the glove up with a ball in the pocket. Good luck.
