So I bought a 16' aluminum boat with a 20HP Johnson outboard motor.  I've never owned one myself so I've never had to trailer one before.  When trailering, should the motor be tilted up or lowered down?  And in particular, is there anything special I need to know about securing the motor for trailering?

Thanks.

Tilt it up.

Look into purchasing a transom saver. Google it if you don't know what that is.

Sounds like a good fishing rig. Have fun!

Don't just look into buying a transom saver - BUY ONE ASAP!

Transom savers are great, especially for larger outboards, but not always necessary for the smaller & lighter 25hp & less 2-cycle motors (4 strokes are much heavier than 2 strokes!). If the very bottom of the motor (the tip of the "fin" below the prop) is less than 10" or so off the ground, then get the transom saver and trailer with it up. If it's higher than 10" or so, I'd just leave it down and in the "shallow water" unlocked position... and don't back up into any inclines, and always launch it with the motor up. You should also tow the boat with it as level as possible, to keep both hitch and motor from scraping the ground while traversing dips in the road, like driveway entrances. Higher vehicles like 4X4 pickups might need a drop-hitch to lower ball height, lower vehicles might need the drop-hitch reversed to raise the height of the ball.

I've been towing boats since the 1970s, only had one with a transom saver (65HP Merc). I now tow a 14' modified V-hull Jon (Tracker Grizzly) and a 25hp Merc with the motor down & no transom saver. The tip of the fin is 11" off the ground when the motor is lowered and boat on the trailer... never had a problem. But it is a short-shaft motor & high riding trailer, the trailer axle is much nearer the rear of the trailer than most, so there's not much "tilting" of the trailer when traversing over dips in the road, and is an exception to what most trailers are. Also, not all trailers have a frame crossmember near enough the rear of the boat to attach a transom saver to (mine would be a real stretch, as there's not much trailer behind the axle). Make sure a transom saver will fit before you buy one.

I see most boats with smaller engines not using a transom saver but I consider it cheap insurance.  Not only does it protect the transom but it keeps the motor from flopping around up and down because I'm guessing it doesn't have power trim on it.  Most of the time even if the motor is locked up or down it still has some play to bounce around but not with a transom saver on it.  I don't use one on my 8hp kicker motor but it has the trim and tilt from a 40hp motor on it so it's not going anywhere but my big motor always has one one it, the motors either in the water or on the transom saver.