Warm up with a longer, lower and more relaxed contact, this will encourage the horse to stretch over their back and will help with the engagement later on.
I like to use a serpentine on a relaxed contact to warm up, as this really helps to get them bending around the inside leg and places a focus on balance from the outset. It is important to keep the rhythm, and balance, through the turns and encourage the horse to bend around your inside leg. Keep the horse moving forward around the turns, but always keep balance in the forefront of your mind.
As the horse begins to warm up you can collect the contact. This will also help develop their serpentines in a more collected frame.
Transitions within each pace such as collecting and lengthening, are great to include in your warm up.
You only want to move the pace forward for a few strides and them bring them back under you. It is very important not to let them run on.
When asking them to lengthen and move forward it is important not to rush them and not let them go on the forehand. Don’t get too greedy with it, control the impulsion and just ask for a little bit at the time.
This is also a really good exercise to help develop the medium trot/canter in horses.