Be friendy, humorous and honest. Play the senior card. Practice interviewing. That is, have many, apply for all jobs that could fit somehow. 90% of the specs in the ad are bogus anyway and are collected and written by people who can't even abstract a desktop icon from a file on the harddisk, let alone acutally know what they are talking about or asking for in a hire.
Display self-worth by not having to prove yourself anymore.
When you're losing your inner game just think: "If you don't hire me, that's your problem, not mine. I'm just being nice to you."
If you're in your mid-fourties, start wearing shirts and perhaps even ties (I'm going to start wearing my first tie soon), along with the matching pants and shoes and maybe a jacket to match. Skip the next 2-3 generations of high end grafics cards or other geek gadgets for a quality wardrobe. Get a good book on dressing well and perhaps pay a professional tailor to give you some advice if you are a total fashion n00b. It may even be time to give those printed t-shirts to the red cross or use them as oil rags.
Get and maintain a good haircut and pimp your grooming skills. Talk smart and less that a usual nerd and keep your voice calmer that you're used to. This all works particularly well if you've already got some gray hair to show. I call this 'the gray hair bonus' - played well it has a solid direct positive impact on your salary.
I got my last job by being friendly and honest and telling some interesting war stories about my times as a developer. We talked for 1,5 hours, had a lot of fun and in the end I got the job. 1 phonecall, 2 short emails (one being the contract for me to review) and a nice long chitchat. They didn't see a single piece of official paper from me. That's how interviews should go at 40+ when you've started programming in 1986 as a 16-year old.
If you're an IT expert you'll get a job, one way or the other. Don't worry to much. Take the edge of age discrimination by being approachable but with a senior aura. Your boss should to feel safer and better understood when you're around, because you're 'the experienced guy' on his team. That works best when you're around his age and are friendly and forthcoming when pointing out flaws in his software production.
My 2 cents.