I assist with admissions for a graduate program at a large University (40,000+ student population). Our program sees over 400 applications per year, first an online application, and then we do in-person interviews. You're probably doing most things right when applying, spell checking your submissions, make sure your references are strong, meet all the requirements, etc..
Things you DON'T want to do - make yourself stand out in a negative way. Any applicant who doesn't submit docs on time, whose interactions with staff are rude/negative/pushy (including reception) - we note and keep track of all of that and you will not be getting an interview, despite what your GPA may be. If you waited until two weeks before the submission deadline to ask a referee to send a letter and they didn't get around to it? Don't call and ask for additional time, there's a deadline and it was your responsibility to meet it. Didn't like the answer you got to an emailed question? Don't come in after seeing our note that we don't do in-person advising, and try and pressure staff into giving you a different answer. Don't, for the love of god, have your parent call or come in on your behalf. There are privacy laws for a reason and we don't want a student who can't ask their own questions. Unsure if your documents reached us? Don't call or email to ask about them; read the instructions we listed that stated to check your online application as all updates will be made there and that we won't take call to confirm. Don't ask to speak to the Department Head or an Admissions Committee member because you have a 'unique' situation. We note everything. We receive enough applications from students who exceed all of our minimum requirements that we can afford to be extremely picky.
The people who do stand out are the ones who did everything that was asked of them; submitted all documents on time, met and completed all requirements, didn't come in when we said not to, didn't make a fuss, were passionate, enthusiastic and had a positive interaction to everyone they interact with.
Don't just be a resume filler. One year in the environmental club, one year in the history club, one year in the robotics club, etc. shows you're just trying to con your way in. Find something you really enjoy and stay devoted to it. Show you're willing to put in work for something you care about, not just go through the motions.
Part-time jobs. In this day and age a lot of kids go to college never having held a job. If you did have a part-time job and can get a letter of recommendation from your boss (responsible, hard working, gets along well with others, reliable, honest), it goes a LONG way.
You'd be surprised how often people use a copy and pasted response to questions, or even their entire admission letter. You can have the best application ever seen, but your chances drop instantly if your admission letter for University X still says "for these reasons and so many more, I believe University Y is the perfect fit for me."
Ok, so go to University Y. Application rejected.