Find the source. Heat rash. Viral rash. Allergy rash. Infectious rash. Scabies. Eczema. Contact dermatitis. Yeast. The best strategy is to determine the source of the rash and address that specifically. General non-specific measures to deal with itchy rashes can include oral Benadryl (diphenhydramine) elixir, soothing oatmeal baths, OTC aveeno anti-itch cream, and prescription steroid creams. Also keep baby cool & use 100% cotton.
Creams. First, you can alleviate the rash by changing your baby's soap and lotion. Try a mild soap such as cetaphil or dove for sensitive skin. Good moisurizers include aquaphor, eucerin and cetaphil. You can also apply anti-itch creams such as calamine or Hydrocortisone 0.5% cream. Both are available over the counter.
Apply lotion. In general, a mild, unscented lotion to add moisture to your baby's skin is a good way of easing an itchy rash. Some lotions such as cetaphil, eucerin, and aveeno baby are good options. Vasoline is also a reasonable option. Some pediatricians will recommend a topical steroid cream such as hydrocortizone, especially if your baby may have eczema. Depending on the age, Benadryl (diphenhydramine) may help as well.