Given that Felltower is a grave of clerics, how do you improve their ability to survive?
With the caveat that this is conjecture, given that I don't personally play any clerics and all the ones I've GMed for have died (or played but once), here are some ideas:
For the Good God's Sake, Buy Luck. It's on the template. Buy it. Use it when you need it.
Avoid Combat. It's hard to avoid combat entirely as a group. But as a cleric, stay as far away from combat as possible. Yes, yes, I know. You need to stay close. You're guarding a flank. You really need to get just a bit closer to cast that buff.
In my experience, that's where clerics get killed. It's better to fail at the spell or not be there to fill in the gap then to be dead. You just aren't survivable against threats that can kill knights, barbarians, etc. of equal point value to you.
Extra HP - to be fair, Extra HP are not listed in DF or the DFRPG on the Cleric template. But generally characters can purchase up to 30% of their ST in extra HP. Clerics start with ST 12, which means 12 x .3 = 3.6 = 3 extra HP. That's 6 points, and leaves you with 15 HP. If you go for ST 13 (see below), this means 16 total HP. 3-4 extra HP might not seem like much but it means 18-24 extra HP needed to automatically kill your PC.
Resistances - A DF cleric can spend 5 on Resistance to Evil Supernatural Powers (+3) or 7 or (+5). Give that a hard think. Up your Will. Up your HT. Be able to resist non-combat supernatural effects. Your goal is to stay up and use your spells and holy might.
Get a shield - Use a shield and a weapon that will let you parry. Get the biggest shield you can carry. DB is your friend, and no amount of DR you can reasonably expect to purchase and wear is enough so maximize not getting hit.
Buff Yourself - If you do expect to get into a fight, or close to one, buff yourself. Shield is critical - learn it and use it. But still, your best bet is to be further than an All-Out-Attack away from the nearest foe.
Those are my suggestions - what are your additions based on what you're read (or played, for my players) of Felltower?