The first thing recommended for picking berries in Finland is the time of day to go. Mornings are best, because the heat of day hasn't fully set in and if it's a weekday the trails are less likely to be crowded. Mornings are also best because as soon as one starts bending his/her back to pick berries motivation decreases exponentially.
The most common berries in Finland are blueberries and strawberries. Of these, blueberries are by far the most common. There is also apparently a berry called a "cloudberry", but I have never seen them and it is altogether possible that they don't exist. To pick blueberries, you want to be as far away from the road as possible. If you can describe your location using the words "forested glen," then you're probably on the right track. Blueberries loathe civilization. Strawberries, on the other hand, are just the opposite. Strawberries yearn to be part of the hustle and bustle of the modern world, but are also extremely shy. They like to grow close to roads but not without the cover of tall grasses and/or other shrubs. There is a patch growing by the parking lot of the supermarket near my dorm room here, as if the strawberries are desperate to see what's going on but too shy to take part in the action. For whatever reason, strawberries also seem to like to grow on an incline. This is probably also testament to the fact that they want a view of what's happening but also the inaccesibility that only a hill provides. In this way they are just like the Finns themselves, shy but always wanting to participate, eagerly anticipating the slightest invitation.
If you are going to pick berries in Finland, be sensitive to the wildlife. Today I saw a man walking with ski poles, huffing and wide-eyed like he had just run a half-marathon, and I was sure to give him a generous berth. Later I thought I saw a black bear standing on it's hind legs but it was just a large Finnish woman wearing a dark blouse and bending over a patch of blueberries, pawing at them greedily.
Your berry picking is over when your basket or strainer is more red than blue, as strawberries are much harder to find and require more effort. Don't be ashamed if you don't find strawberries on your first attempt. Try a few phrases in Finnish directed at the forest or feign that you genuinely don't care about finding them. When you least expect it, there they will be. Also, stick to the roads. Unless of course you're looking for blueberries, then stick to the glens. Any glen will do, just make sure to watch out for ski-pole wielding Finns.