Good posture and walking style. Shoulders straight and slightly back, chin high but tucked a little, neck straight. Good posture makes your clothes look better too! Get suits tailored to fit. Take notice of your hands - when your arm are relaxed by your sides, are your thumbs facing the wider world, or hiding against your thighs? Thumbs in indicate rounded shoulders - some time with a physiotherapist or biomechanic will help to correct it. [I've just had a look at your gallery - you look like you're doing fine, but I've left this here to help others]
Speech: look into elocution DVDs or books, but the main factor is to enunciate. Dropping Gs and mumbling are habits to shake. Train yourself out of filler speech - y'know, um, err. A moment of thought before you start speaking helps you work out exactly what you're trying to express, and the best way to do it.
Read. Educate yourself. Newspapers, books, magazines, Wikipedia. Everything's available online these days anyway. If you don't understand a topic of conversation, ask for a brief synopsis. Or do some research, find out about the sophists, or gymnosperms, or doppio ristrettos or whatever it is that's come up.
Other than that? Common decency and good manners. Please, thank you, excuse me. Hold doors for both sexes. I think being a gentleman is primarily about being a good human. Work on that, and the rest will follow.