Start with a mild detergent such as Dawn and water. If there is grease build up, follow with mineral spirits or naphtha. In extreme cases, I use a product that is chemically similar to Formula 409, wiping with it and then rinsing with clear water.

Note, though, that chronic long term exposure to acidic body oils can degrade finish into goo. Remove the goo and there is no finish there (only the goo that used to be finish). At that point you should at least re-seal with a finish product.

Here is a link that might be useful: Saving the finish