The biggest challenge with fresh garlands is how to prevent them wilting.
Store in a refrigerator or a cool garage.
Then just before decorating or displaying an undecorated garland give it a good long soak in the bath tub submerging it over night and up to 24 hours in cold water to cover the greenery. Having taken it out, shake off the excess water.
Once Hung:
If it is undecorated, you can remove it overnight for a re-soak if you wish.
A more popular way is to spritz your garland with cool fresh tap water on a misting setting.. Go for the cut ends. If the stems have become dry and wooden recut them so that the water can be absorbed.
If you mist the front, the greenery will still absorb water but a little more slowly.
The secret of the trade is anti desiccant spray which locks in moisture. This can be bought online.
This can extend its life up to 4-5 days longer.
Direct heat is the enemy of a long lasting garland. Try to keep it out of hot airwaves, warm / hot hanging locations, and sunlight.
If using lights go for LED's as they do not generate any heat: If winding lights through on a string, avoid traditional incandescent bulbs because they pack a real punch when it comes to heat and can wilt your garland.
With a cool environment and moistened stems your garland should last a few weeks.