Once your plants start to produce berries, you may have to take extra care to protect them until harvest time. Picking frequently can help reduce any losses, but there are several situations where you will need to take some extra effort with your plants.
When it comes to weather, the biggest potential threat is hail. Unfortunately, it is also a difficult problem to protect against because you usually don't have a lot of warning. If your local weather is predicting hail, you can cover your plants with any sort of cover to protect them from the impact of the ice.
Bird netting (also mentioned below) can also help, particularly because it can be left over your plants all season. The very fine netting with large holes won't offer too much protection, but a finer netting can help reduce hail impact on your plants.
Thankfully, blueberries are very cold tolerant (even the Southern varieties) but they can be damaged by any unseasonal frost that strikes after the blossoms have come out. Berries themselves are usually fine after a frost but flowers will drop off and then produce no fruit. Bushes that mature early are at the greatest risk because they bloom so early in the season. If you are expecting an unusual frost, you will have to try to protect your plants the night before.
A sprinkler system is the most effective way to protect against frost though and it can be used on small or large scale crops. The water needs to be run continually all night to prevent any freezing of the plants. If you can't have a sprinkler, then your plants must be covered. A light plastic sheet will work though larger bushes can be difficult to cover this way.
Scarecrows are not that effective, but any kind of noisy or flashy gadget can help to startle away the birds. Shiny tin foil plates are a common home-made tool that actually can work quite well. Hang at least 2 of them by a string together, loosely enough to easily move in the breeze but close enough to each other that they will make a noise when they knock together. Two or three of these around your berry garden should make a big difference in your bird population though they can be a little noisy if they are also near your house.
A more fool-proof way to keep birds away from your berries is with a covering of netting. You can buy light-weight bird netting for this that can be a valuable investment. It can be draped directly over larger bushes, but with smaller ones you should use poles or stakes to hold up the netting so it doesn't pull on the plants.
The netting will have to be taken down at harvesting time, but otherwise can stay up over your plants all season long.
Aside from birds, you may have to protect against larger animals such as squirrels, rats, raccoons or even deer. Standard fencing can be helpful but most of these pets can (and will) climb a fence with little difficulty. You can use the bird netting to help protect your plants, but it is a fairly light-weight product that will not stand up to the strength of a determined raccoon. Chicken wire might work better if you have to put a cover over your plants.