Proper Warm-up
Establish a consistent daily routine that works for you and keep the overall purpose in mind - loosen up those muscles. Area to touch on include:
Lip Flexibility use lip slur exercises over the entire range.
Tone Quality long tone exercises in moderation to avoid fatigue.
Breathing
Intonation use a tuner for reference.
Ear Training focus the lips and ear.
Review Fundamentals ( i.e. articulation, dynamic ranges, etc. )
Expand Playing Range both up and down.
Warm-up time may vary - minimum of 5-10 min is necessary.
(Information and photos courtesy French Horn Information Centre.)
A list of warm-up and fundamental books from Lanette:
Bai-Lin-Lip Flexibilities for all Brass Instruments
Farkas, Philip-The Art of French Horn Playing
Pares, Gabriel-Pares Scales for French Horn