mignonette I'm so glad I got a question!
We are open every day of the week from 9am to 10pm (community in the day and young people 8-18 years after school as well as clubs - karate/asian womens group/community football etc) and for young people we have both generic sessions and targeted sessions.
So the generic sessions have a 'open house/hang out' feel where they can access all the facilities like inside and outdoor sports spaces, table tennis, pool, air hockey, tv (usually on music channels!), games systems, tuck shop etc and we are around to facilitate activities and build relationships which helps to get them to the targeted session which we can let them know about when we know their interests.
Targeted sessions include a 'creative arts' session (dancing, poetry, art, singing, songwriting and playing instruments - we have a recording studio too), a girls group, a sports night and junior/senior nights where we look at age appropriate issues.
I co-ordinate all the youth work (write the programmes, organise/recruit/manage staff, budget, promotion etc) and lead on sessions within my specialty (sex and relationships) so I'm pretty free to do whatever I want to improve numbers of contacts - I'm on maternity atm but from the start of the year to when I went off (April) we had a weekend residential in the Lakes doing water activities, a theme park trip, a trip to a recording studio, various guest sessions including the police (anti-social behavior chat), an ex-con (drugs chat), a professional free-runner and a football at Wembly trip.
We advertise on facebook (the centre has its own page and is 'friends' with young people - also a good way to keep an eye on any potential situations to manage for example fallings out, cyber bullying, grooming etc), we do outreach work where we'll walk around the town centre trying to get them to stop being a nu science/annoying people and come into the centre, leaflets in schools and have the top hit on google for various search terms...other than that its word of mouth I think.
We do have to compete with online gaming (numbers dip a little whenever the next big game is out!) but we rely on various methods to keep numbers up - we have parental support which helps (I guess parents think its 'healthier' or more 'wholesome' to be 'playing' than stuck in front of a screen), we offer incentives in terms of trips/qualifications (mainly AQAs and first aid type ones), its relational work so kids often come in to see us and say hi, we often have sessions where they can get information that may be difficult to obtain elsewhere (sex/drugs/drink/relationship advice), its somewhere to see their mates where they wont get in complained at or moved on...
Sorry thats a massive post isnt it?! Hope I've answered what you asked!