doe-eyed has some good suggestions. Meditation can be a great way to help you and your alters learn to switch at-will, or even make switching easier in general. Calming your mind and focusing on yourself can bring to light little "warning signs" of a switch that you can remember, and then you can be more prepared for a non-at-will switch.
Personally, I used to force switches with methods such as music, like doe-eyed mentioned. It's not an easy learning process, and it's not a perfect process. At that point in time, we were just looking for something that worked. It's not something I would suggest using as a regular method for at-will switching. It's good for like emergencies or something, but self-induced triggers should never be done continuously or repeatedly for too long. They have their appropriate time and place.
I would suggest looking into meditation. One thing that may help you, especially with any littles or any scared alters: make yourself feel as safe, relaxed, and comfortable as possible. Surround yourself with items that make you feel safe like stuffed animals, pillows, favorite blanket(s); perhaps wear favorite clothes or favorite jewelry; maybe have calming/relaxing/happy music on in the background; perhaps have a favorite/comforting snack or drink (like a milkshake or your favorite juice or tea, etc.); whatever you can do to help yourself feel safe and relaxed, do it. Usually the better you feel, the more it helps your alters to feel better too.
Other than meditation, you can try using music to help bring alters closer to the "surface"; this is different than listening to triggering music. Do you know any genre they like, or favorite songs they have? Listening to songs that have a positive connection to them might help. You can also try this with food they like, movies they like, clothes they like (wearing them before they come out), doing activities they enjoy, etc. Another thing you can try is writing notes, then meditating to see if you can get whoever you wrote for to answer; or even writing about them- I know sometimes for me if I start writing about an alter, they'll come out to write about themselves (usually because they don't want me "getting it wrong"); you could try doing different things that keep your focus on them- such as looking for a picture or drawing a picture of what they look like; making lists about them (likes, dislikes, hobbies, etc); I think you get the idea.
I certainly hope you find a way that helps you, and that switching becomes easier for you.

Best of luck with this!