If you have completed a DMP then hopefully any defaults will drop off your record soon - if not then write to any creditors who haven't altered things and ask them to remove the defaults as you will have paid off your debts in full.
Your credit rating will slowly build up again.
If you have been in an IVA then this remains on your file for six years from the date of acceptance. Again, any defaults registered after that date should be removed.
However, your credit score will probably be very low. Having a mobile phone contract, keeping your bank account up to date and mortgage if you have one will all help.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
I've been in a DMP for 2 and a half years and personally i don't think it's effected my credit history too much. I have a phone contract and never have any problems renewing that. I also have a credit card that is not covered under the DMP and they have just increased my limit without me asking.
It seems to me that as long as you keep up all the payments under the DMP your not shown as defaulting and therefore not adversely effected.