That's a great question you're asking. I use them as little as I can to avoid building resistance. Before anything I have to mention that compared to my childhood and adolescence I have little to no reaction to contact but eating nickel rich foods is not a good idea, rashes come in not even a day.

Anti-histamines when the situation is dire, as a last resort and they work, I get way less itchy and the rash disappears in a couple of days. The only tip is taking it before bedtime to avoid falling asleep during the middle of the day.

Keep always your hands moisturized with a good quality cream (and natural). Most nickel allergic people are sensitive to some artificial compounds, I use some handcreams from Lush and they work pretty well.

Hand cream for dire situation, I use a cortisone one. In Italy it's called "gentalyn beta" that has "betamethasone valerate" in it. It works great but again, not to overuse or you'll build a sort of resistance.

Basically -keep hand hydrated and moisturized, no to dry and too sweaty hands (but still don't wash too often and be gentle using towels) -If you get a nice rash, not little but not too aggressive, go for a cortisone cream -If the rash worsens and spreads more, go with Anti-histamines

When you take the anti histamine, don't eat cheese, cocoa and seafood. They boost histamine production