Mimes and Rhymes
Making sure your class is aware of the sounds and the shapes of certain letters is the best place to start. Perhaps begin with a handful of letters each week and hold them up in class on a daily basis until they become familiar with them. You might also choose to send a list home of the letters you are looking at each week (or every few weeks), so they can practice at home too. As with everything, repetition tends to be the key with young learners.
Once your class is familiar with a healthy group of letters, you can start to talk about the form of each one; whether it's curvy, straight, has a tail, a hat or a dot over it etc. This gets them thinking about the actual look of the letters. Hold up one letter at a time and get them to draw it in the air with their fingers. This is one of the best activities to start to introduce the form of letters as well as to refine small motor skills. For observational purposes, this is also valuable for you to see which children need more fine motor skills practice.
One of the best ways to help children remember how to form the letters is with a rhyme or short phrase. Even if children recognize an "m", for example, they won't necessarily know where or how to start writing one. Read Write Inc. has a phrase for each letter of the alphabet, but perhaps the reading system you use has a different one, or you can easily make up your own.
If we take "m" as our example again, you might have a picture of a girl called Milly next to two mountains (Milly forming the first downward stroke and the mountains making up the humps of the "m.") The phrase then goes "Down Milly. Over the mountain, over the mountain." As you repeat the phrase, make the action over a large m, so the children can see what you mean.
Try combining these activities together by getting the class to say the phrase aloud and at the same time as drawing their "m" in the air.