Yep great idea for a thread. I do a fair bit of video using multiple platforms, drones, gopro's, DSLR's and pro level video cameras. From what I see there are a number of things that folks (including myself) cock up so I'll run em down.
Firstly understand your equipment. Yes the Gopro is pretty impressive out of the box on auto settings but if you want to get the best out of it then you need to know the different settings and how they work. I suggest going to
Abe Kislevitz page. This guy is THE gopro guru. He does a fair chunk of the promo video you see for Gopro and has a heap of good information in regards your camera. Some of the stuff is old and refers to earlier models but most of it is still very valid.
Know what you want at the back end i.e.what you want the finished product to look like. This doesn't mean you have to script and story board the whole production but if you have an idea of the story you are trying to tell then you'll have a better idea of what to shoot. This includes B roll. What is B Roll? B roll is SOOOOO under rated and under used. It is the stuff you put in between the main shots to give context. You can use it at the start and finish as well as in between.
Lets say you are shooting a race. If you shot the race on it's own it kind of lacks context. Where is the race, who is racing, what sort of race is it? B-roll can give you this. You could open with shots of a sign at the start of the race or riders getting their bikes ready. During the race footage you could include the scenery where the race is held, even wildlife nearby, shots of the pits area and don't forget the crowd. At the end of the race you could get footage of the podium, then bikes going into the back of the ute and people getting into the car and driving off. These are all basic examples and super easy to do but they give your video a much better feel and help tell the story. B-Roll is important and you should get absolute shitloads of it. You don't need to use it all, far from it but the more footage you have to work with the better chance you'll get something that will work.
The next one is probably the biggest mistake Gopro users make in my opinion and is actually a post production error. Filming the whole run and then just banging on some music and uploading it. BOOOOOORRRRRIIIIINNNNNNGGGG. Watch any action sports video and you'll notice that generally 3-5 seconds is the longest shot you see accept for the slowmo stuff, even then they generally are still only short punchy clips. Don't get me wrong there are times when longer shots are called for but generally keep your clips short and to the point. It's easy to fall in love with your footage but you need to be merciless and chop it. Then when you think you've clipped it enough go back and do it again. You'll be surprised how much this can make your edit pop. Especially if you can sequence some of the clips to the music. Which leads me to the next issue.
Music is super critical. You need to choose the right music for the action and try to edit to the music as much as you can. Change clips on a hi note or a beat, use slowmo in slow sections of the song, speed up again as the song kicks back in. This takes an ordinary video to the next level. Downhillers are the worst for this. They bang on some insanely fast death metal and it just doesn't generally suit the action. I like a bit of that music so it's not a taste thing here it just doesn't fit unless you can edit to it which is hard. Find something a little slower but still heavy if that's your flavour and edit the clip to the music. It will look waaaaaay better.
As for gimbals, I use a Feiyu Tech WGS gimbal with my Session 5 on a chesty mount. This works really well and gives very smooth footage even on big hits. I also suggest using the gopro stabilization as well and you'll have a really good chance of getting silky smooth footage. My favourite trick at the moment is to put it on my back and film riders in close behind me doing big jumps. This looks amazing and is really under used in my opinion. Gopro's looking forward are ok but when you look back at someone you can really see what they are doing and it really pops. Give it a crack.
Anyway that's my 2C worth I hope it helps.
Cheers Scott.