Making friends and influencing people

I don't understand the diplomacy/alliance/ aspect of the game, leading to constant mid-game issues.

In all games that I have run through (using different government and traits), around the mid game point several alliances start to form, yet no matter how much I put into trading, embassy's, opening borders etc, I have never had an invite to form, or been invited into an alliance.

Latest example is an alliance of 3 parties, 2 say yes, one says no due to a conflicting government. All other potential alliance partners are at war with others so that option is out of the question. It's now looking like this alliance is getting ready to take me on, they are 3 small vassels between them, which for no apparent reason, hate me. I'm waiting for the demands for my planets to come, along with 3 fleets rolling in against me.

Is there a diplomacy for dummies guide anywhere? What am I missing or doing wrong? I feel like I'm missing out on something very basic here....