Listen to your dialogue.
What I mean by that is do whatever you need to in order to "hear" it. Read it out loud if you need to. I find that I have developed a lot of practice and I can actually "hear" it while typing. It's a matter of writing a lot.
Unlike anything else in your story, dialogue is not meant to be communicated silently as written words. That is, while it must be communicated that way, it is an imitation of sounds we make in order to communicate audibly. The very best thing you can do to make that seem more realistic is make it audible (or imagine it as sound if you can).
Listen to real conversations. Go the other way and visualize what you are hearing as written words with quotes around them. Pick out a random snatch of words from a conversation your mother in law is having with your wife on the back porch. Think about what that would look like on paper. Make your fictional dialogue imitate those sorts of patterns.
You will quickly realize that gaps, spaces, and silences communicate a whole lot in real speech. There are ways to indicate this on paper, like ellipses. In many cases, the thing that has the most impact in a bunch of dialogue is the pause, the silence, the hesitation in the middle of the words.
One of the most basic things I almost always do is tell the reader who is saying what only for the first couple lines of dialogue, then, if the conversation makes sense and the characters have a distinct enough "voice", it should be totally clear from context who is saying what. If your characters all sound the same, you have other problems. I'll usually go back and remind the reader who is saying what one more time after about 6 or so lines of dialogue, so I don't tax them too much. I don't want to make anything challenging, but I want to remove "Tommy Said" and "Said Mary" as much as I possibly can, because I think they look ugly on the page.
Listening is key to other aspects of your writing too. If you can mentally "listen" to the way your writing sounds, whether it is dialogue or not, it becomes much easier to write in a flowing, conversational way that will keep readers' attention. This helps all aspects of your style in general, so it is never a bad thing to read what you write out loud now and then.
I would also add that once you settle into a convention in a novel (like my first couple lines, then four to six lines, then another reminder use of character names in dialogue) don't mix it up, stay with a pattern that the reader has gotten used to. Try to stay consistent with how you are handling dialogue.
I personally find that the best dialogue is "sparse" in details other than what is being said. I don't mix too much action and dialogue, generally. I also don't think of that dialogue as everything that those particular characters ever said to one another during the time my story is covering. I think of it as excerpts, highlights, like the Cliff Notes and most interesting tidbits from all the dialogue that would have taken place between them. Surrounding the dialogue will be the context, the action, the explanation (when necessary). That way, when readers see those quotes, their "ears" perk up (so to speak) and they pay attention. You don't need to include every little thing your characters ever say to one another just like you don't need to include every little thing your protagonist does in his day.