what is the best practice between an Agency and a client in order to maintain a long fruitful relationship

That would be: Continuous connection with the client Respect Understanding and a clear vision of both parties (regarding the kind of business between them) Simple! yet challenging in a way.

provide your client with the the update knowledge and confidentially of their sensitive information and data d,t miss use their personal information for personal business advantages..............
don,t miss use his insider information...................which is called money laundering

From my almost 2 decades of experience, I can name 3 things: Communication Collaboration Commerce Without these three, no business relationship (or any other relationship) will prosper.
These key aspects in turn need both parties to be honest, have integrity, mutual trust, understanding, and respect.
I don't think I need to explain the first two.
But with the third, I mean money.
The reality of business is trade.
If the financial aspect is not suitable to one of the parties, then that party will always feel cheated or at a loss, which is not a good foundation of a relationship.

Best practice between the agency and the client to maintain the relationship is a direct relationship in a personal way with each client individually and varied tools of phone calls and direct mail to feel the client that the agency know very well personally and customer contact nature of personal contact and try to find out individual needs and try to satisfy even theup the client with the agency to the stage of full confidence and loyalty.

10 top tips for good client relations This list of ten tips for good client relationship management was produced for the Design Business Association by Jan Casey, an independent consultant and advisor to the design and marketing industries.
1.
Do everything before the client asks You have managed projects before, but your client may not have.
Capitalise on this by supporting them through the process.
2.
Agree the relationship procedures Agree the main points of contact between you and the client.
3.
Explain your project practices Have open dialogue about your practices and tools of communication, including a clear plan of when and how stages of work will be delivered.
4.
Put in place failsafe internal systems Plan presentations, briefings and meetings in advance to avoid looking unprofessional.
Don’t send people to meetings unless they have a clear, predetermined role: it looks expensive to the client.
5.
Be in control of finances Always have a clear awareness of the financial situation across all your projects.
6.
Know and understand your client Learn about a client’s business beyond the remit of the current project.
7.
Develop the client relationship and business Get to know as many client staff as possible and keep them up to date what your consultancy is doing.
8.
Be aware of how you conduct yourself Listen carefully; be confident and honest, but not arrogant.
9.
Ask for feedback To continue improving, find out what you are doing well and where you’re performing poorly.
10.
Go the extra mile Don’t settle into a routine – push yourself and surprise your client.

In addition to the various points that others made, I feel the fundamental requirements are -Communication -Trust -Transparency -Mutually agreed terms and conditions or policies -Goals

I think mutual respect is an important aspect of it.
I've worked with clients directly as a Creative Director (as part of a company) and as a Creative Consultant (privately) and I have found that the grovelling and self deprecation that some agencies practice to keep getting work from a client is shameful.
If I respect my client and in turn my client respects me then there should be no need for any uncivil behavior.
Treating your client like they are a part of your establishment and in turn showing the client that you can be trusted is an excellent way of avoiding having to lie to the client about your deadline meeting ability.
Regardless of how much good I did in my capacity as a creative the Account Manager / Sales person was the main reason why we would lose these good brownie points with the client because either they over-reached their own teams ability (whereby you will eventually burn out your own team) or will lie about our capabilities thereby painting yourself into a corner.
The same goes for clients who forget that they have a big event or a conference and suddenly dump their owes onto your shoulders in hopes that you could save their posteriors.
I don't mind doing this once or twice a year...
but a constant domino action of forgotten deadlines just means that my performance and that of my team will suffer and our work will not properly represented.

Being transparent and providing constant updates!

always strive to create win win situation for both parties.

If i am a client ---- 1.
I would not like to get formal e-mails.
2.
If a client seems not so interested about us i would rather take a side road than to go with direct road.
3.
Clients are always think that whenever a marketing person come to him he will always say positive words regarding his product.
In that case i think surrender the negative features before you say the positive...I am sure he will agree with you 100%....But if you tell him positive before negative it may not create any difference.....

نعم