Beer in a growler typically lasts for 24 hours or less, depending on how full the growler is, how many times it's opened over that period of time, and how efficient the seal is on the lid. Are there ways of prolonging the life in a growler or at least seal it properly?
Unfortunately, there's not much one can do, at least using only household products.
While filling, using a counterpressure bottle filler would maximize the longevity of the beer as well as filling the beer while it's very cold, near its freezing point ~29 degrees F (so that its CO2 is maximally dissolved), if you have a choice in the matter (likely not).
Once opening the growler over multiple sessions, however, it's not the ineffectiveness of the seal (using a swing-top growler should do just fine) as much as it is the growing amounts of air that get sealed within.
There exist several products funded and developed via Kickstarter you might want to check out: