Unfortunately, not all of us are fortunate enough to have an innate ability to stay sweat-free as soon as the temperature starts to rise - even if we are just sitting or standing somewhere. Add to that any kind of light exercise, a little pressure/restlessness and it can feel like someone has tipped a bucket of water on you.
I've always found that the best way to get around this is to avoid complicating your outfits. Casual looks are by far the coolest and least suffocating - regardless of whether or not you take note of our advice - so this is usually how I dress in the warmer months.
My selection of summer outfits is usually based on color, fabric and fit. As ridiculous as it may sound, some colors show off your unattractive perspiration more than others, especially gray. I also find that bright colors in general tend to fix everything, or generally avoid them.
Although many of these problems can be solved by adjustment. For a single season, the most slims of your shirts, t-shirts, polo shirts, jeans, chinos, etc. are not your best friends; anything that clings close to your body simply invites warmth and contact, so opt for clothes that have a slightly wider cut.
Fabric is also extremely important. Heavyweight chinos, velvets and the like are designed for winter - so are thick fabric tops. Lightweight layers will be much more comfortable and much more suitable for warmer days, preventing you from overheating while keeping you cool from sweating.