My top tips:
1. Take a breath before an h. If you have a sentence with a word beginning with h - for example, I'm going to the hairdresser - take a tiny pause and a tiny breath before hairdresser so you can do a little exhale to aspirate the h.
2. If a word begins with a vowel, link it to the word before. For example if you want to say I need some fresh air, you should imagine you are saying I need some freshair. Don't leave any room for an h to sneak in.
3. Practise saying I'm hungry and I'm angry. I'm hungry has a little pause before the h. When you say I'm angry you should imagine you're saying I mangry. You also need to open your mouth really wide for the a sound, like you are going to bite into a huge apple.
4. Beware of words that have a silent h: hour, honest, honour, heir, vehicle, exhausted. Just imagine there is no h and your pronunciation will be correct.
5. Practise, practise, practise. Reading aloud for 1 minute a day and really concentrating on the h's is an excellent way to improve. If you can stand it, record yourself.
When I first met my husband, he put extra h's everywhere and dropped all the other h's. As he often has to do public speaking in English, he really made an effort to practise, and his h's are nearly perfect now. Except after a few glasses of wine, but that's all part of his charm...