Never use any of those witty pick up lines- They do not work anymore. Using pick up lines can be the biggest turn off to almost any person out there. Same pick up lines are used by millions of people out there on a daily basis on dating sites and offline therefore is a very cheap and common way to ask someone out.
Don't ask just tell- People mostly tend to ask someone out for a date by saying this - "would you like to go out with me?” Now when you say that you leave the decision on the person you asked which makes them feel superior to you as you left the final verdict upon them. Instead of saying "would you like to go out with me?" you should say "We are going out on Saturday and I’ll pick you up at 9". Do not give the other person a chance to decide and be spontaneous. More often than not people find it harder to say no to a dominant person.
Don't act desperate- Even if you are you should pretend that you are not. No one likes to go out with a desperate person. Every one likes a person with a positive attitude. Learn to believe in yourself and your potential. Being desperate gives a very strong indication to the person you are asking out that you have been rejected in the past and that's why you are in a state of desperation.
Your posture shows it even before you talk- Body language is a strong indicator of where your level of confidence stands. Some people get so nervous even with the very thought of asking someone out that they start shivering or get a shaky voice. You should learn to stand tall and should act very comfortable when asking someone out.
Don't make it a big deal- Most people consider it very important and do tons of preparation as if they are going for an examination. Asking someone out for a date is not rocket science therefore does not require any preparation whatsoever. Rather preparing makes the situation even more worse as many people tend to forget what they had to say. Always be in the moment and learn to act at that very moment. You should be very comfortable and confident with yourself when asking someone out.