The company will always have more power, more resources, more leverage, more information than you do. Technically, you're "equal" in the eyes of the law, but you're not like, equal-equal, you know, only dumb people or the corrupt would genuinely say that.

So. You (or me or anyone) as the worker is in a position of weakness, from day one, until the last day, and beyond. It's just like it is. In any battle, the weaker party will try to compensate. The tools of the weaker party have traditionally been underhanded tactics, subterfuge, what-have-you.

You can increase your power to tilt things in your direction. To do that, you can increase your knowledge and information - talk to people in the industry about how much they're getting paid for the type of work you are doing, so that you can price yourself properly in the salary negotiation. Talk to people and learn the dark underside of working at company X, or under boss Y, or with technology Z, etc. That way you can avoid personal hell. Research the reputations of companies using online sites with employee reviews etc. "Stalk" companies you're interested in, record how often they re-post the same job ad, to gauge how desperate they are to hire. Talk to your predecessor for tips on what to look out for, ask them how much they were paid, why they left etc. Arm yourself with info.

This is basically the easy, free part - it costs you absolutely nothing, it just requires you to a) communicate and b) spend time to research. So that stuff is free, if you're not doing it, you're being dumb about the whole thing and honestly, no one should listen to your complains about being the weaker side (not you-you, of course, but just anyone who whines but does zero research and info collection. The other ways to increase your power are more difficult. Collect more resources - personal wealth in all its forms - so that you are not desperate. People reek of desperation, so they lowball themselves in the salary negotiation. You would do much better in the interview, if you honestly don't need the job at all, and are ready to walk away at any time. It's kind of like dating in that way. It helps if you have not only physical wealth, but "life" (?) wealth as well - if you have a wife/husband and a family, this gives you certain resilience and a reduction in desperation, you wouldn't be as depressed and that helps earn more. Ultimately, you want to have savings. You will notice that once you have some money stashed on the side, you change as a person. The desperation is gone.

Ultimately, you can gain more power through time management - be aware of corporate cycles, the right and wrong time for certain things like raises, promotions, relocations, etc. know that and use it to your advantage. Always use your performance review to renegotiate salary. Always use any bonus cancellation etc. as an opportunity to extract something else instead - flexible working hours, more vacation days, something. Just use the momentary weakness of your boss, whatever guilt they may feel for having to dangle and snatch away the bonus, and shamelessly leverage that to extract something else in return. Ultimately, if you put money squarely off the table, and negotiate other things, the chances of your boss saying yes to those other things are much, much greater. Companies are greedy and stingy. Tell them you don't want money per say, and be amazed at what amazing other stuff they give you as they foolishly believe those other things are worth "nothing". Two months of vacation just this one time this one year or whatever, may change your life completely, meanwhile your boss will feel all smug and happy "look at what good boss I am, I spent zero dollars and made all the savings in the world" and be all giddy inside for giving you two months off. Remember, you are nothing to these people - use it against them - I am serious, just use their own greed, $-stinginess against them, put money on the side for a second, and genuinely see what else is there for your grabbing.

So yeah. It's not "fair", you will never be equal with the company, but if you arm yourself with knowledge and information, stave off desperation through personal relationships, love, family and a bit of savings, make use of every performance review to renegotiate pay, and use your boss' weak moment of having to deliver bad news to you, against him/her and shamelessly extract another type of concession or perk - and you'd do much, much better than all the other losers who only whine but do nothing as they expect someone else to do their negotiating for them like the weak underpaid losers they want to remain forever. It doesn't have to be you.