I'm a middle and felt like that. As a result, we have made a deliberate effort to give all our dc individual attention and to give the middles attention when the eldest and youngest are present. They can be real attention-grabbers, and often the middles just give up and shut up.
One of the things dh and I do is to go away with each child at learn once every year. It can be anything from an overnight camp to 3-4 nights travel abroad, depending on season, school, finances etc. That way each child gets a good chunk of one-on-one time with each parent.
We also try to spend time with each child individually every day, even if it's only 10 minutes, and not just talk about things that they have to do.
When eldest and/or youngest are monopolising conversation at the dinner table, we will let them run on a bit and then redirect the conversation to middles. This has become extremely difficult in recent years as they've both turned into computer geeks, and dh is a software engineer. Oh, and they also have the autistic tendency to need to take a subject to the bitter end.
Unfortunately, while our middles are aware that we deliberately give time and attention to them, they still feel either squashed or overlooked.
Our dc are 22y - 14y.