I'm an infrequent trailer tower so have to get comfortable every time I do it. I borrow my next door neighbor's F150 which is a really nice towing rig. I rent either U-Haul tandem car haulers or their tandem 6x12 utility trailer. I've also rented a 20' tandem float, much like you've pictured.
The lesson I have to re-learn every time I draw a trailer is to look further down the road. Looking up close will have you fighting the wheel to keep the trailer centered on the pavement. Looking further away helps reduce the tendency to constantly be correcting and over-correcting.
The comments about leaving extra space are right on the money. Some idiot is always going to want to fill that space, so just get used to it. Signal your turns well in advance, and I make a habit of flashing my brake lights (just enough brake pedal to light up the lights) when decelerating or coming to a stop - it wakes up the idiot behind you.
With respect to tongue weight, I make a habit (doesn't matter if it's one of the bigger rental trailers, or one of my small utility trailers at home) of trying to pick up the tongue while the trailer is unloaded, then trying to do so while the trailer is loaded. This gives you a good comparison, a way of guessing how much weight you've got on the tongue.
If loading a tractor with a 3pt implement attached, be prepared to swap the load end for end in order to get the tongue weight correct. On my last haul (B7510 with cab & front-mounted snow blower) I found I could achieve excellent balance by loading the tractor backwards on the trailer.
As has been mentioned previously in this thread, always tie down your implements, even if they are attached to the tractor. Tie them down like they were standing on their own. This removes the possibility of them shifting, gaining momentum as they move, and jerking your tractor around on the trailer when they hit the limit of their travel.
Lastly, once you're loaded and ready to go, stop at the end of the driveway and check your tiedowns. Go another mile or two, and stop and check again, especially if you've been driving over a bumpy local road. If you've done it right you'll have the wonderful confidence that comes from knowing your load is properly tied down and not shifting. If you haven't done it right, you'll learn quickly what doesn't work when tying stuff down on a flatbed.
