the piece i am doing for my youth symphony tryouts has a lot of slurs in it. i can almost do them but i keep getting extra notes in the middle. any help?

Sluring on the french horn is next to impossible, and that's with years of private study and years of practice.

That being said here is the best advice I can give you (but you really need private tutoring or private lessons to be good on the french horn - very few exceptions). To be good at slurs you need to practice slurs. Don't get the misundestanding that the slur is a smooth connection to the note up above or the note down below - a slur is not a gliss. If you were to slur on the piano (since you have experience playing the piano) how is that done? It is the same way on the french horn. A slur is a quick change from one note to another note without articulating the note.

Ex 1
start on C just below the treble clef, go to C# (tonguing).
Repeat but this time slur (or go from C to C# without tonguing). Then go C to D. C to D#, C to E, C to F, C to F#, C to G, etc all the way to F (top line of the treble clef).

Ex 2
Same as Ex 1 but start on a high note like F on the top of the treble clef and go down to C, or lower.

Technique : When going up in the slur give a quick kick with your lower abdominal muscles. Just quickly kick them in, it will help get the higher note, it gives a little assistance to your diaphram muscles.

Be sure to use your proper vowel shapes for the range each note is in. Look at what key your pieces are in and practice sluring on those scales.

well, i'm not really sure how I learned, just don't distinquish your note. Hold your tounge still and sort of hum/buzz. good luck!

Er, you hum your slurs? Weirdo.