Ok I want to help you out because... well I just want to! But I'm new on here and I'm NOT a girl.
Firstly do not to the gift thing. Seriously going back with a gift just because you left something in the restaurant is not good. You want to go for the meal method.
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Also you might want to change your aims as it helps with confidence. If I were in your situation then my aims upon going in the restaurant would be to find out three things you have in common and to make her laugh three times or whatever. Don't think about dates or numbers because I find that just puts pressure on myself. That might not be the case for you but I like to take small steps.
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Right...
I think the best option is to go back on the same (quiet) night with ONE other friend. I actually think it would be better with a girl actually. If you do then make the pizza the priority but do mention to her that you want to strike up a conversation with the waitress. Ask her to smile if you do start and maybe she can add a little to the convo too. This will give the impression that your just mates, that your single. If it's a guy then tell him about the cute waitress too, and make sure he keeps his mouth shut. He can be looking at the menu or something!! I wouldn't go in with more than one friend.
Mention that you REMEMBER her as the nice (or at a push cute) girl who brought the pack back out. Instantly your telling her that your interested in her as you've remembered her from a week or two. This isn't creepy. By nonchalantly slipping in the word cute this reinforces it. With that she'll hopefully be taking your order etc and you've got a minute to ask her a few questions. Obviously ask for name, perhaps age and where she's from, but most importantly go for questions where you'll be able to say "Me too". This establishes that your similar. Eg "So are you a student here?" you can say "Oh me too I'm a student at...". If you work at the weekends tell her, worked in a food place before etc etc. You've really got to do this in a friendly but playful manner, make it look as though you do this regularly to girls, that you always chat to waitresses. Don't do it with a really concentrated look on you. You've got to look fun and playful, she's at work so add in a little fun! Tease her about her outfit or how she has to act so "professional"! Get her out of the workplace!
If it goes well then you can occasionally ask her over with a cheeky grin "Excuse me waitress can I have some more salt?" or whatever. BIG SMILE whilst you say it. Or even "Excuse me Miss Waitress...." That's bound to put a smile on her face.
However don't make her the main focus of you coming into the pizza place. Stretch the meal out, chat with your friend (< two more reasons why you should pick a girl to take!!), order dessert and make sure she doesn't catch you staring at her, that will be really bad. Remember filling your gut up and spending quality time with your friend is much more important than any girl who you've only known for half an hour.
Now if everything has gone good so far (ask your female friend if your unsure), and if you've hit your previous aims (as I talked about in the first paragraph) then it's number time!! Don't even BOTHER getting it on the second week as you'll have little chance, you want to do it on the first proper contact with her. You want to show her that you have confidence and don't fear rejection. Coming in again the next week will mean that she's tagged on to you liking her and you'll lose the whole "mysterious/interesting" side that any guy has when girls first meet them. You've got to seal the deal on the same night that you opened it!
Best time will of course be when your asking for the bill (not when you've given it to her, that is too late). Personally I would say something jokey like "So I hope you've enjoyed having us as customers" or "Hmm I'm not sure if you deserve a tip or not....". I would ask her "What time do you get off?" followed by "Oh so how many days do you work here then?". With that I'd get in. Again I'd keep to the confident routine and say "Oh so you'd be free to join me next Friday to see a film then?" or "So are you free Wednesday? It's just I would love to take you out and Wednesday would definitely be the best day?". I suppose you could just ask "can I take you out sometime" but I've never done that. By using the former I think you've got more room for manoeuvre. Look at her face when you say this and you'll be able to gauge her instant reaction. Anything positive then say "Cool so can I get your number?", anything unsure then you can say "I'm being serious you know!" or "Arr are you going to give me answer then?". If she says she's got a boyfriend then say "Oh that's unlucky". If she says no or looks negatively then I'd just say "Oh your no fun!" or "Woah your missing out then!". If she quickly changes the subject then change with her and don't ask her again as it's clearly a no. Apart from saying "Thanks" with a big smile when she brings back the bill I'd leave it at that, I certainly wouldn't hang around talking to her anymore. Save your chit chat for the date! So take your lady friend and leave and she can congratulate you when you get out
Oh and one more thing if you do get her number then DON'T TIP! If you don't get her number then DON'T TIP! If you didn't ask for her number I wouldn't bother going back as it will be sure sign that your into her and not the pizza and that just isn't what women like.
I have posted so much now that you'd better do exactly what I say and in a weeks time come back and tell me how it went. Or else!
Good luck.