With regards to using Jugs/Flasks/Gulps/Drops of Karma most efficiently, the largest possible bonus at this time is 95%.
To obtain this bonus, you'll need to:
- Use a Karma Booster for 50%
- Touch a "+10% Karma and +10% Experience" Banner
- Eat an ice cream food item for 5%
- Be under the effect of the "+15% Karma for 24 hrs" guild buff
- Have an item with a Karmic Infusion in it equipped (15% bonus).
Altogether, these bonuses will combine additively to give a 95% bonus.
- 95% is not a fixed cap, it is simply the largest bonus available at this time. It is possible that more Karma Modifiers will be added at a later date.
It's worth noting that although the Outmanned buff claims to add an extra 33% to Karma gain, it does not work for liquid karma items (and as Raven Dreamer pointed out, it actually does not work at all right now). The Outmanned buff no longer claims to add a Karma bonus (Thanks for pointing this out, Brian).
- Thanks to Metemer for pointing out that there is now a 10% Karma Banner.
Source: Guild Wars 2 Wiki page on Karma Modifiers