Save Garrison for the partially neutralized allied capture point rather than a "healthy" one to take advantage of its regeneration function.
Garrison can't be cast on neutral turrets.
Garrison will interrupt all enemy champion from capturing a point, with its long range, it's useful to get just the time to save the point.
When using Garrison on the enemy capture point, all players should focus on defeating the enemy champion. Because if the player starts neutralizing the capture point, it will stop attacking, defeating the purpose to activate the spell.
Casting Garrison on the capture point that is already under the effect of another one's Garrison (regardless of which side) will overwrite the previous one. Try to avoid casting it by more than one ally before its effect fades.
The negative effect of Garrison on enemy capture point is aesthetically indicated by a stream of pink halos descending on top of the center of the capture point.
Garrison on allied capture point is ineffective when you are severely outnumbered. Because the opponents can have one champion to start neutralizing (disarming the turret function in the same time) while the others to attack the lone defending champion. In such condition, it is preferably to cast some safe harassment and retreat immediately instead of defending and wasting the Garrison on it.
Although Garrison substituted Fortify in the Dominion mode (prior to the latter's removal) and shares a similar icon design, it does not have a global range and can only affect one, rather than all structures in the battlefield.