DHEA is commonly used to combat HIV and other immunological deficiencies such as lupus, but precious little has been done in terms of clinical testing. At the first sign of any illness, consult your vet . Early aggressive treatment is the rule. Also, watch for any sign at all of weight loss. Spend at least 15 minutes every day in direct contact with your cat.
Toys are great, too.
Keep all areas where your cat spends time as clean and disinfected as possible. This includes where the cat eats and sleeps, the litter box, windowsills, perches, and anywhere he or she spends much time. I strongly recommend clump-free type litter, and experimenting with brands until you find the one kitty likes best. Personally, I use one that is free of dyes and perfumes. Anemia can easily kill your FeLV cat and is in fact a leading cause of death.