A thing I was told to look for in interaction is when conversation hits a high point is when you leave. Basically you want to build her "buying temperature" to a point where you know she is totally feeling you (showing SOIs, kinoing you, getting sexually aggressive, kissing etc) then when all signs are green that she is totally focused on you and is loving your company you make an excuse to leave. If she is really into you at this point she will whine, get upset, pout, make excuses to keep you. and then THAT IS WHEN YOU NUMBER CLOSE.
Sounds like you went for the number without building her "buying temperature" enough. Try working on watching for escalation in her interest in you (SOIs. Her getting sexual and responding well to your lead and touch) when temperature is high, and I mean she can't keep her hands off you and is giving you the bedroom eyes BAM hit her with the excuse. "I gotta go see my friends/got work in the morning/whatever"
If you did it right she will get upset at you trying to depart, it's actually really cute when you see a girl get upset that her new favorite potential boy toy is leaving her
. Then hit her with..
"Aw don't give that face it makes me feel bad (smile). I tell you what; put your contact in my phone so I don't lose you."
I never say "give me your number" simply because hot chicks hear that phrase and it involuntarily re-launches ***** shields (seriously, I've talked to some hot chicks who hear "Can I have yo number" several times a day several days a week. It's like a killswitch in most hot women's heads!)
So yeah basically keep flirting and doing your thing. Have fun interacting but keep escalating sexually. In the back of your mind watch for Statements of Interest and her returning the sexual progression. When her buying temperature is visibly at an apex, try to exit with an excuse. If she tries to get you to stay, seems upset by you trying to exit go for the number.
Now if you are interacting and she doesn't seem to be feeling you (you should be able to tell if she is interested withing 5 minutes of interaction) THEN NEXT THE FvCK OUT OF HER! Smile, saygoodnight and walk away with your head held high. Good luck brotha.