1. Warm up the group
The best smile is a natural smile. Best way to capture a natural unforced smile is to warm up the group right before the photo. There are a few games you can play to warm up the crowd. Photography Concentrate has some great tips for warming up your subjects.
- The Whisper Game - Ask everyone to whisper a funny secret and a silly joke to the person next to them. A natural curiosity happens when we hear secrets, that'll bring up the energy level and lead to a natural smile.
- Ask Happy Questions - Great for couples. Ask them to recall a happy moment together. The power of recall is very powerful tool that stirs up emotions long after the event has happened. Use happy moments to bring out your subject's best.
- Fake Hard Laughter - Ask everyone to give you their most ridiculous laugh. This will get people warmed up and excited. And you'll most likely get them laughing at each other's laughs brining out their natural laughter smile.
2. Don't ask for a smile, warm up to it
It's hard to time a well placed smile. But you can get everyone on the same page by asking for a smaller smile then asking them to smile bigger and bigger until you get the right moment. This makes it easier to time the smile you want right.
Another way is to ask for the opposite. Photography tips site SLR Lounge recommends for single subject photos, you can ask the subject to make their most serious face, then capture their smile when they burst out into laughter when then can't hold it in anymore.
3. Good angles to capture faces better
Some angles are just more flattering than others. Digital Photography School recommends using different angles higher than eye level for children. This emphasizes a lot of childlike qualities bringing in the youthfulness by focusing on their face, and slimming the body. Using an extra high angle has the added benefit of reducing shadows around the nose and chin that makes people look more serious/omnious.
Low camera angles tend to enlarge the body in ways that are not flattering to most people. It creates an authoratative feel, but not a great angle for capturing smiles.
4. Good lighting angles for faces
Improved Photography has some great tips on lighting that reduces unflattering or distracting shadows and a handy PDF about lighting angles. Avoid distracting shadows by using a fill light or moving the subject further away from the wall.
Adorama Learning Center has some excellent tips for outdoor photography. They recommend telling the subject to turn their head and taking the picture form a slightly different angle to avoid the harsh sun. When working with direct overhead light, like the midday sun, you can soften the harsh shadows by using refectors or the flash on your camera. They have some great sample shots, worth following the link and checking it out.
5. Poses that help bring out smiles
When taking pictures with very young subject. Doing silly things with the nose is a great way to excite a natural smile from your subjects. The pictures will not only have that natural smile, but a nice adorable touch.
Stylecaster has some great tips for posing for your photos to bring out your best smile. They recommend touching the tongue to the roof of your mouth while smiling to reduce smile wrinkles, and angling your face slightly to make your smile look more 3 dimensional.