Writing: How to Write a Letter of Advice

Expressions to advise

I strongly recommend that ...
I believe the best course of an action is ...
If I were you I would ...
I would suggest that ...
Why don’t you ...
It would be a good idea to ...
The best advice I can give you is ...
What you should do is ...
I think ...
I really think you need to/must/should ...
How about …?
It is usually a good idea to ...
My suggestion/advice is (to) ...
Why don’t you …?
You could try …
You probably/definitely/really should …
Have you tried …?
I would (strongly) suggest/advise that …
If I were you, I’d …
In my experience, … works really well ...
It’s generally best/a good idea to …
One idea is to …
One thing you could/should/have to do is …
The best/most important thing (to do) is to … /… might work / … would probably work /… (always) works for me ...
If I was/were in your place, I’d …
If that happened to me ...
In that case ...
If I had that problem, I’d …
My (main/personal) recommendation is/would be …
You’d better …
In this (kind of) situation ...
I (would) always recommend/advise …
If I were in that (that kind of) situation, I’d …
Have you thought about …?
If it were me, I’d …
Make sure you (don’t) …
The sooner you … the better.
Whatever you do, …
Your only option is to …
You have no choice but to … 
… is worth a try/trying ...
A (self-help) book I read recommends …
A piece of advice from … that I’d like to pass on is to …
A wise man once said …
As the proverb says, …
I can’t recommend … strongly enough.
You should …, no doubt about it.
It might be an old wives’ tale, but …

Closing remarks

I trust you will accept this advice...
I hope this will be of help to you...
I would very much like to know if this was helpful...
Hope this has helped...
Let me know what happens, etc.