First, let me just say that balloon arches and garlands are a pain in the rear end! While I do enjoy putting them together (I mean, who doesn't love balloons?), sometimes they just seem to have a mind of their own and do not want to cooperate. Just kidding, but it kind of feels like that sometimes. They will spontaneously pop and scare the bejesus out of you and just when you think you have them perfect, they will shift, shrink, or pop. Especially, outdoors when the wind is blowing or if it's really hot and humid outside.
Second, do not claim to be a balloon arch expert. I've had to create quite a few, and I feel like I'm pretty good at putting them together, but there are many companies that focus on balloons that are WAY better than me. However, I have learned a few things along the way, and if you're a DIYer like me, this may help you out a little.
#1 Keep them cool.
The heat and humidity are not your friend when it comes to balloons. Being in the deep south, it's always ridiculously hot and humid which makes outdoor balloon arches a nightmare! Balloons will typically last for days if not weeks when stored inside and climate controlled, but they will only last hours outside in the heat and humidity. I try to keep them cool as long as I can. I wait until the last possible minute to blow them up and transport them and I will keep my car running with the AC on for as long as possible if I have to transport them. That also means making sure my car is cool before I load up the balloons. The arches in the pictures below were all outdoor arches, in the middle of summer. These pictures were not taken long after they were completed and you can already tell how much they shrunk and shifted from the heat.
#2 Build the base first.
What I find to be the easiest for me, is to build the base on the balloon strip first, and then attach all the special and fill in balloons once the base is in place. Most of the arches/ garland I have made have at least been around 16ft long. Even with moving just the base, it never fails, balloons will fall off or pop. I like to get the base up first, then come back with the different sized balloons or specialty balloons and design.
#3 If using balloon arch/ garland kits, always get extra supplies.
I have personally found when using the kits that you can buy from stores or off of Amazon, that they usually don't include enough balloons or balloon glue. I will always buy either 2 packs to make 1 arch/ garland, or will buy extra balloons with complementary colors. I also ALWAYS run out of balloon glue. The little 100 dot packs that come in the kits just don't go that far when you are building more elaborate designs. I highly recommend purchasing extra balloon glue dots to have on hand. And really, the more balloons the better. I've never seen a balloon arch and thought, "that has way to many balloons". But I have seen them that looked kind of dinky.