Acquiring wishes
Possible sources for a wish:
     A wand of wishing. These cannot be recharged, but you can keep zapping
the empty for one last use.
     A ring of djinni summoning. If it's blessed and you (u)se it, the
djinni will grant you one wish.
     Drinking from pools will sometimes (rarely) grant a wish.
     Starting with 0.9.9gamma7, there is a spell Wish / Divine Intervention
that can be cast for a wish.

What to wish for?
     First, don't try to wish for wishes: it just doesn't work. As of gamma
8, wishing for a spellbook will get you one, but that doesn't mean you'll
be able to learn or cast the spell. Wishes for wands of wishing yield Wands
of Wonder, and wishing for rings of djinni summoning will get you a brass
ring--a used RoDS. The wands don't recharge, so wishing for scrolls of
recharging or other items that recharge wands won't get you any more
wishes.
     Second, don't wish for any of the artifact items, because you won't
get them. You have to earn those.
     Third, spelling counts! The game does not yet parse for spelling
errors or near-misses. Don't get too specific, either, because it might
kill a perfectly good wish. You can't specify number, beyond using the
plural, the blessed/uncursed/cursed status, bonuses, or the color. Well,
okay, you can specify the material *if* the item is available in multiple
materials. So "eternium tower shields" is fine, but "3 blessed fireproof
blue plaid gloves [+4,+1] {Ch +6}of waiter summoning, size 5" is a foolish
wish, indeed. If you want it blessed, rustproofed, or otherwise improved,
you'll just have to do it yourself. After all, you are suppossed to be some
kind of resourceful adventurer, aren't you?

Now, what should you wish for? It depends on what you need/want most. If
you have a favorite skill that the character is missing, you can wish for
the skill (except for alertness and healing). Just type the name of the
skill; I like Food Preservation, Detect Traps, and Herbalism. If you need a
particular stat improved, you could wish for it directly
(Strength/Intelligence/Willpower/Health/Beauty/Charisma/Mana/Alertness), or
wish for the appropriate potions and bless them, which usually yields a
better improvement. Before gamma 8, potions of gain attribute were popular,
because you could get several, bless them all, and raise all of your stats
several points; now the wish only yields one potion.
Some people have favorite items they like. Popular weapons are whips of the
snake, swords of sharpness, phase daggers, daggers of death, maces of
destruction, or maces of disruption. Others prefer clothing or armor such
as girdles of giant strength, seven league boots, eternium plate mail, red
dragon scale mail (or white/blue/black), long bows of accuracy, bracers of
resistance, or several others (let me know yours, I might include it).
Some have favorite or rare items, like anvils, spellbooks of (favorite
spell), amulets of order/balance/chaos, rings or amulets of teleport
control, rings of ice, or toolkits like cooking or fletchery sets.

Probably the most wished for item is the scroll of chaos resistance. Make
sure to ask for the plural, scrolls. These can extend the effective time
limit of the game by removing some of those nasty corruption effects.
Others wish for the poitions of cure corruption, but the blessed scroll is
more effective than the blessed potion.