Putting in the effort to not be complacent is sometimes hard, but spending the time to think through each and every unit on the board is worth it—if you can predict how the board state is likely to flow, you are better positioned to influence it.
Beyond that, tactics are an obvious place to start. Diplomacy's tactical rules are neither very complex nor super simple, but sit somewhere in between. In that, it requires some effort to figure out and understand on a deep level, but it's relatively straightforward to get to that point. I have a list of diplomacy resources that I like to share with people I'm trying to help improve. Take a look at the tactics section here:
An Index of Diplomacy Resources.
Bot games are a good place to learn the basics and practice some of these tactical skills. 1v1s too, given their speed and relative simplicity (as in, there's only one opponent). However, nothing beats playing against, well, other players. Only then can you get true experience with the breadth of the game.
For improving in 'real' games, playing more obviously helps, which doesn't necessarily mean more games, but also more time devoted to each one. (That is, sending better and more frequent press, putting in the thinking work to figure out what to do and where to focus.) More games does help, especially to experience more varied situations at different points in the game, but only if you can devote sufficient effort to each one. Gunboats do tend to require less time, which can be a good way to get a feel for the strategy involved with different situations.
Finally, read articles written by other diplomacy players! Most high-level players I know got started by reading articles from the Diplomacy 'Zines', whether the
Diplomatic Pouch, the
Diplomacy Archive, or others. Nowadays, there's been a bit of an explosion in new modern productions, whether swordsman's excellent
Brother Bored blog, the
Diplomatic Briefing weekly newsletter, or various YouTube channels. (I don't follow any since in general I tend to find them overly long and rambling, although some are certainly good! Perhaps others can share which ones they like.)
Little of the advice written about diplomacy should be taken without question, in my opinion (perhaps even this advice), but they do certainly offer noteworthy insight into the thought processes that others bring to the game, and perhaps some of it can be useful.
But to finish, perhaps let me just stress that there is no one way to be good at diplomacy. Some people are tactical and strategic masterminds, and the diplomatic part is a way to implement their plans. Others are manipulative chessmasters who seem able to get people to do what they want, almost as magic. Nor is there even necessarily only one way to define "good". It is a game after all. Perhaps the greatest good is to enjoy it