Without failure, every time I visit a park on a sunny day I see guests with beet-red skin. If you're a person like me who burns easily and wants to avoid getting charred, you should wear sunscreen with SPF of at least 30. Don't forget areas such as the scalp, thighs, and hands, which will surely get exposure when riding coasters. I recommend putting on sunscreen before you arrive at the park, reapplying around lunchtime and once more in the afternoon. Most major parks offer shaded areas, but if you plan on riding a lot of coasters and it's sunny, significant sun exposure is inevitable.
You can never drink enough water at a park on a hot day! Being dehydrated can contribute to things like more frequent grey-outs on coasters and heat stroke. Your hydration options include water fountains, paying $4 for bottled water, and/or buying a souvenir cup. Each comes with pros and cons such as cost and loose article policies on rides.
When I spend a full day at an amusement park, I usually walk between 8 and 12 miles. I'm always surprised that some guests wear flip flops or uncomfortable sandals. Keep in mind that rides may require people to remove loose footwear and step on dirty/hot concrete. My go-to amusement park shoes are sneakers that I wear every day. Also note that your shoes will probably get wet on water rides.
Many parks allow guests to enter a few minutes early. Arriving early (I suggest parking 30 minutes prior to the listed opening time) will allow your party enough time to clear security, get your bearings, decide where to go first, and have a "game plan." Furthermore, I've found that the first hour of operations is usually the least crowded time of the day. Take advantage of this opportunity if you can.
Zipper pockets are the best way to safely get around loose article policies. I have 4 pairs of these types of pants, and each cost me $20-$30. It's worth the investment to store belongings such as cellphones, wallets, car keys, small tubes of sunscreen/hand sanitizer, etc. with the peace of mind that there's no way these items will fall out during a ride.
Depending on your circumstances, you might benefit from paying extra money to skip the lines at parks. I outline criteria that I use when determining whether I want to splurge here.
Even I have my limits. It's a good idea to take a break after riding 5 or so coasters in an hour (which only happens if the park you're visiting isn't busy or you skip the lines). Some activities that I do in between my coaster-riding sessions are eating/drinking, using the restroom, taking photographs/videos, sitting and resting my feet, riding less intense coasters or transportation rides, and shopping for merchandise. If I'm too ambitious with coasters early on, then I usually regret it later in the day, so I've learned to be realistic and take breaks often.
This tip might be too obvious. When possible, I schedule my visits outside of holidays and summer weekends, which are typically the busiest days at amusement parks.
There seems to be a general consensus in the coaster community about which rides are good, bad, overrated, underrated, etc. I had ridden well over 100 coasters before I followed mainstream coaster personalities, so I already had a pretty good idea of what my preferences were. Had it been the other way around, I almost certainly would've been swayed and indoctrinated into thinking that all Vekoma clones are bad, the slightest bump makes a coaster rough, and every RMC is automatically the best coaster in the park and top 25 material. Some people even "pre-rank" roller coasters before they've ever gotten a chance to ride them. I think that's silly. No matter what other riders say about a particular coaster, it's important to ride it yourself and come to your own conclusions. They don't have to match what others think!
This action is very dangerous and could get you ejected from the park. It's not worth injuring someone to take an onride selfie or video. Plus you would almost certainly have to replace your phone if you lost your grip. Even though this tip seems like common sense, I have personally witnessed a dropped cellphone traveling 70+ mph at an amusement park three times and it's really upsetting.