Moving units away from a border is a pretty good sign that you're happy with each other.

Remember that lots of things take 3,4 phases to come through. That is, if you support someone in spring 1901 they might not be sure you're serious until mid/late 1902.

Most importantly - check the orders logs for failed orders that might not show up on the map

+1 figles, always check orders logs if a unit doesn't move.

Def agree with figs (+2).

I usually try to stay away from straight support holding. Instead, I'll support a move that I know isn't being made. For example, if I'm Ger and I want to signal my desire for peace with Fra, but we have units crowded around the Lowlands (Fra: Pic, Bur, Bel / Me: Hol, Ruhr, Mun), I would have Hol support Ruhr, but Ruhr would support Pic to Bel, and Munich, Bel to Burg.

I am a little curious about how people would read the above move?

The above would be if I wanted to attack to the south. If I wanted to go north I would have Ruhr S Bur to Bel and Mun S Pic to Bur.

I'll be totally frank, I would not have picked up on the direction of the attack, or even that it signals an attack, because I'm fairly new to gunboating. But that is brilliant, and I do believe I'll be adopting it for my own games.

Yeh I'd pick that.
Blocking up a region is a good way of applying for peace, although can become an issue if/when you try to DMZ the area.

The other key one is remembering to repeat orders. eg if Germany is giving France Bel, he may have to support in 2 or 3 phases before both nations pick the same unit to move/support