The easiest way, IMHO, is setting up a humidifier for the entire room.
It's winter here so without a humidifier I would be sub 40%. I run an AIRCARE 6 gallon humidifier that keeps the entire room at 45-60% depending on outside considering. Low humidity can impede calcium uptake and cause faster transpiration than necessary.
Since I exhaust and intake into the same room, running a large humidifier compared to running one inside the tent is much less complicated. Eventually the air mixes enough that the RH is slightly higher in the tents so then I can veg and flower adjusting exhaust speed for temp/rh control.
Otherwise you can
Mist the walls
Use towels with buckets to wick
Sacrifice floor space in your tent for a humidifier
There's the elaborate method of using an intake fan in combination with your exhaust to regulate air flow.