The most important thing IMO is to try and keep the moisture level as even as possible, without the soil becoming overly wet or overly dry for any prolonged period. Soil that is ammended with water absorbing crystals could help even the moisture level, but also the soil should have a conditioner such as perlite to loosen so that it drains well and does't stay soggy too long, especially near the bottom of the container.
Spring and early summer will be pretty easy in the N.Y. area because temperatures should be moderate, with fairly cool nights. During the hotter part of the summer, the watering schedule will be more demanding, maybe even requiring a couple of waterings per day. You may make that task a little easier by buying or constructing a self watering bottle that plugs into the box and slowly drips water over a prolonged period.
The last thing I might mention is trimming back the petunias from time to time. When the plants get very long, trailing far over the edge of the planter, they obviously require much more water than when they smaller. Trimming the petunias will cause them to branch more, becoming fuller and they will bloom more as well. So trimming them will give a triple benefit.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.