As I see it there are the following options:
- Take a level of a class with Con save proficiency at first level.
- Ability Increase (Con).
- Diamond Soul - (14th level Monk - not really viable)
- Aura of Protection - (+Paladin's Charisma bonus, Also affects nearby allies)
- Tides of Chaos - (Advantage, Wild Magic Sorcerer ability, self only)
- Bend Luck - (+1d4, Wild Magic Sorcerer ability used on another)
- Dark One's Own Luck - (+1d10, Fiend Pact Warlock, self only)
- Portent - (Replace a bad roll, Diviner ability)
- Transmuter's Stone - (Proficiency, Transmuter ability, but you have to give up other powers)
- Inspiration - (You must have Inspiration, game mechanic)
- Lucky - (Reroll bad rolls, feat)
- Lucky - (Reroll bad rolls, 1/2ling ability)
- Resilient - (Proficiency, feat)
- War Caster - (Advantage, feat)
- Focused Conjuration - (Can't be broken, Conjuration spells only, Conjurer ability)
- Bardic Inspiration - (+1dx, Bard ability, thanks BlueMoon93)
- Bless - (1d4 added to save, requires concentration... so defeats the purpose unless you can use it on more than one player.)
- Foresight - (advantage w/o concentration, but this is a 9th level spell)
- Gaseous Form - (advantage, requires concentration, can't cast spells - this is for skedaddlin')
- Holy Aura - (advantage, requires concentration - 8th level spell, and super costly)
- Shapechange - (Change into a creature with proficiency and even advantage with Con Saving throws - 9th level spell)
Magic Items & Boons:
- Belt Of Dwarvenkind, MANUAL OF BODILY HEALTH, Ioun Stone of Fortitude, Blessing of Health - (Increase Con by 2)
- Amulet of Health - (Con is set to 19)
- Holy Avenger, CANDLE OF INVOCATION - (Advantage - while in effect)
- Cloak of Protection, Luck Blade (also reroll), Stone of Good Luck, Ring of Protection, Robe of Stars, Rod of Alertness - (+1 bonus)
- Staff of Power - (+2 bonus)
- Don't get hit! - High AC, or non concentration spells that mess with getting hit (mage armor, mirror image, feats to wear armor, levels of Cleric/Fighter), Up your Dex
- Use Cover - Thanks BlueMoon93 & nitsua60
- Don't be seen - Invisibility, Illusions, get stealth skills.
- Get out of combat - Mist Step, Dimension Door
- Go Prone against Ranged attacks - thanks BlueMoon93 & nitsua60
- Warding Flare (Light Domain Cleric)
- Defensive Duelist feat (finesse weapon required)
Controversial or Edge Cases
- Temp hit points
- Make potions for spells that require concentration.
Are there other options that I have missed?
There are not a lot of ways to get to the concentration saving throw buffed. Which is to be expected in 5e, but I am tired of losing a spell first or second round, so I am trying to build up a list of tactics / ideas on how best to hold onto that 5th level spell I just cast.