I feel I've had quite extensive experience of going to concerts and festivals. My first was 1992, my most recent was 2018, and that includes large festivals, medium-sized venues that hold a few thousand, and smaller, more intimate venues of perhaps 200 capacity.
In my experience, I've observed that people go to concerts with different goals and expectations. The front row is usually people who want to intensely listen, watch, and hang on every bit of stage banter. The back row just want to be near the bar, drink, and enjoy the show vicariously. The middle section is a mix of people, but among them are those 'sneakers' you mentioned - those who want to be at the front row, but arrived late, so try to push in - and also a few who want what both the front and back row have, so constantly flit between the two. After a few drinks at the back, this latter group suddenly decide that the front row look like they are having all the fun, and now they want to be part of it. The problem is that those on the front row believe they have the right to be there because they planned to be there - they arrived at the venue crazy early, waited in line for hours, and then made a dash to claim their position at the front. So when those annoying middle people suddenly come bouncing in with their half-full plastic beer glass sloshing around, it all seems very unfair.
This may sound like I don't sympathise - this isn't true at all, in fact, I'm very much a front-rower myself and find it just as annoying. The trouble is, there is no such thing as "concert etiquette". There are just too many different kinds of people at a concert with different aims to be governed by a single set of unspoken rules. You can't expect someone whose purpose is to drink and dance to respect the space of someone who wants to stand statically and listen. And as for the 'sneakers' - well, they paid the same ticket price as you, might be just as big of a fan as you, but perhaps their personal circumstances and family/work commitments didn't allow them to queue up for hours like you. In their mind, they are just as entitled to the front row as anybody else.
I'm glad you asked how to communicate with such people rather than ask what you can say - in a noisy concert you are not going to be able to verbally communicate much. In fact, you will be lucky to make eye contact with anybody. You will have to send the message that you are not prepared to move by other means.
In my experience, the drunken dancers will quickly move on if you just hang onto the crowd-control barrier and don't budge. Their attempts to get to the front are opportunistic, and if they aren't given the opportunity, they won't persist. Once they realise their drink has been sloshed everywhere they will return to the bar.
The 'sneakers' (I do like that expression, I'm going to use it from now on) are like a shadow though - they'll hang behind you waiting for you to give them an inch, then they'll take a yard. Again, just don't budge. This sends the message that you are keeping your place and are not prepared to give it up. Sooner or later, they will find a way to the front, as someone will either let them in accidentally or on purpose, but that shouldn't affect you if you keep your position. Also, keep your eyes on the band - don't show your annoyance, as that will make it personal. If you make an enemy of someone trying to push in, they might make it their goal to get your position. If they can't phase you, can't get your attention, and can't move you, then they will find someone else to push in front of.