When it's warm and sunny people are generally happier and more relaxed which makes it easier to capture relaxed portraits of them. Also, as the days are longer you have more time to shoot which means you can get home from work and still spend a couple of hours taking portraits in your garden or out on location. Better still, the light during later afternoon/early evening is warm and low, the perfect time to shoot portraits.
A focal length between 70mm and 150mm will help create a shallow depth of field, isolating your subject from the background. You'll also create a more flattering portrait working with these focal lengths as parts of the face, such as the nose, will be less prominent thanks to the perspective that's created from shooting further away from your subject. You can use prime lenses too. A lens like the 18-270mm f/3.5-6.3 will be ideal, as will the HD Pentax DA 70mm f/2.4.
Here are some top tips for better images:
Shoot in the shade - Shooting in direct, bright sunlight won't produce flattering portraits but you don't want to work in an area that will cause the light to appear dappled such as under a tree canopy that has lots of space between the leaves. Instead, find a shaded spot where the light's more even and they won't end up squinting. Try positioning your model in a doorway, under an arch or under a tree canopy (just make sure the light's not dappled) and shoot with the light behind them.
As your subject will be darker than the background it can really help make them 'pop' from the image, especially if you make use of exposure compensation which is something we'll cover shortly. If you find your shot could do with a little more light, try using a reflector to angle light where it's needed before reaching for the flash.
Back Lighting - Be careful if you decide to use a strong backlight as it can fool the camera into underexposing your subject. Instead, try metering from your subject and let the background overexpose. You can use flash if you prefer but this can leave you with a portrait that doesn't look as natural. To further enhance the feeling of summer, lighten your subject's skin even more by using centre-weighted or spot metering with exposure compensation of around +1 or +2. You'll probably have to experiment though as the brightness levels will obviously vary.
White Balance - Try taking a few test shots in auto but if you're in a place where there's a lot of greenery your camera can think it needs to compensate for it and you can end up with a pink-ish cast on the skin. So check your LCD before you fire off lots of shots and switch to manual if needs be.
Background - If the background you're working with will help add interest to the image then use it but do remember the photo is about your subject and not what's behind them so the background shouldn't be distracting. Most of the time a simple, out of focus background will work the best. Don't let what your subject is wearing steal all the attention either – keep it simple. If you want to add some foreground interest try blurring foliage into an out of focus frame or use something more obvious such as a window or door frame and position your subject within it.