Your first line of defense will be conformal coating. There are various options here. Easiest to apply for DIY electronics would be in spray form (such as Electrolube HPA200H spray)
However, if you have an RF component and/or RF circuity in your PCB, you should be careful with conformal coating. As the frequency goes up and impedance requirements gets tighter, conformal coating becomes harder and harder to use, because it alters the RF circuit behavior. Also, obviously, you should not be coating certain sensors (pressure sensors, humidity sensors, etc).
Your next line of defense is your enclosure. Ideally, all your connectors should be IP67 or IP68 rated, and you should place your PCBs in an enclosure which is itself IP67/IP68 rated. This is usually achieved by using a gasket and designing the enclosure with a proper gasket opening.
If your device is going to be exposed to sunlight, you should make sure that any active IC in your system will not exceed its operating temperature. Metal enclosures exposed to sunlight will heat up considerably. You may want to think about a cooling solution, based on your power dissipation (enclosure with fins, etc).
And if it's a sealed container OP may want to consider a bag of silica gel desiccant. Depending on what else is in the box (or what the box is made of) they might want a vapour phase inhibitor.
However, if you have an RF component and/or RF circuity in your PCB, you should be careful with conformal coating. As the frequency goes up and impedance requirements gets tighter, conformal coating becomes harder and harder to use, because it alters the RF circuit behavior. Also, obviously, you should not be coating certain sensors (pressure sensors, humidity sensors, etc).
Your next line of defense is your enclosure. Ideally, all your connectors should be IP67 or IP68 rated, and you should place your PCBs in an enclosure which is itself IP67/IP68 rated. This is usually achieved by using a gasket and designing the enclosure with a proper gasket opening.
If your device is going to be exposed to sunlight, you should make sure that any active IC in your system will not exceed its operating temperature. Metal enclosures exposed to sunlight will heat up considerably. You may want to think about a cooling solution, based on your power dissipation (enclosure with fins, etc).