The poor priest class seems to have been rather neglected on the wiki, and since I primarily play a priest, I thought I would put down some things I've tried or plan to try bringing back to the priest class.
Cross class skills in Priests
The mage is the most logical choice for a priest to cross class to. Priests can use almost all mage equipment, importantly staffs, and with the addition of some of the hefty fighter class armour can become quite formidable. MP and damage will drop slightly, but the added defense and HP can greatly increase your up time. With the free addition of Divine Armor, you can almost tank and cast at the same time. Whether dealing damage or providing status effects, Priests with mage skills are bound to be an asset.
Victim Return is practically a necessity for any serious priest. The amount of time and effort saved by this one little spell is quite remarkable. Max MP Increase is probably worth bringing over too, if you can spare the space.
Priest cross classed with thief can make a devesating PvP character. Traps, Trap Release, Warzen Blow, Stealth, coupled with Stun Smash, Torchlight and support magic can make dealing damage and preventing it a walk in the park. No more sneaky trap thieves ambusing you!
Alternatively, some of the more supportive abilities such as Decoy or Medicine Shower will greatly increase your party's lifetime. Stealth is a great help for getting rid of some of that passive agression from healing.
Most fighter skills carry over poorly to priests. Priests simply lack the attack power and speed to do much damage with these abilities. That said, Defend Stance is always helpful for keeping you alive while things are whacking you, and Self Heal can save time healing yourself so you can move onto other people, although because of the large number of points required to get this to a decent heal it may not be worth the effort.
As a priest, you should always go for the AoE spells rather than the single target ones, provided there's an option. Ignore Protection and go for Mass Protection when you can. Put levels into Heal Circle rather than Heal. It might cost more MP when you're by yourself, but a solo priest that is not specifically built around being a solo priest simply does not last very long at all. You're more use in the long run if you wait until the next level and grab something amazing and put multiple levels into it.
Note that I haven't really tested any of these in much depth. In all cases, I strongly recommend Victim Return if space allows it (although grandfathering High level Revive or Resurrection can serve roughly the same purpose)
Aim for Magic Aura and a high level Divine Armor. Holy Light is an excellent choice, since Light elemental damage does increased damage to pretty much everything and is INT based. From Mage, carry over your choice of damaging spells (Fire Blast is recommended for Lawful alignments and Earth Tremor for Chaotic for the decreased casting times). Spells with incidental status effects are perfect for giving your party an edge. Ice Burst and Ice Pillar can give you a quick reprieve while other spells cool down. Quick Cast will increase your damage and healing rate significantly.
Most of the skills in the mage's Aggrevate skill tree can be bought over for both boosting your own party's power and decreasing the enemy's. Action Bind and Hard Bind are life savers if enemies are vulnerable to them. It's essentially a long range stun smash. A high level Armour Melt coupled with high level Attack Aura can let your fighters decimate monsters. A good Decoy will of course work wonders and buy you precious healing/blasting time. In the priest branch, aim for good AoE support abilities such as Mass Protection or Attack Aura
From thief, carry over your choice of traps + Stealth. Warzen Blow is also a good choice. Since you need to be Chaotic for this build, the special effect in Armour Melt can potentially work wonders. In the Priest trees, aim for Stun Smash and Fast Break, as well as Mass Protection and Attack Aura. This should allow you to do large quantities of damage before they even get to you, and should they survive that long they will meet a heavily armoured, high attack powered priest who can stun them into next week, exploit the stun, and heal themselves completely mid-battle with no difficulty.
Again, Stealth and your choice of traps. Trap Release will stop anyone else blasting you unexpectedly. Torchlight will not only stop nasties from sneaking up on you, it will also paralyse them, which can be enteraining to say the least.
If desired, from the thief tree bring in Make Medicine and Medicine Shower for a quick and easy heal. Otherwise, grandfathering skills like Middle Heal, Heal and Heal Circle can save skill points that can go into the less used skills like Body Cure or Mind Cure. If you can manage, Holy Breath being grandfathered in will save you from putting anything into lesser buffs at all and you can focus on healing. Unless you are likely to regularly party with a fighter with a shield, there is little point in getting Repair Shield.