This limited study (although certainly better than nothing) have found that people are more open for up to 14 months afterwards (and therefor more creative due to decreased inhibitions as RobotRipping & NeverKnowsBest said). That study was in the USA and it is positive that there is a small push for studies to be done in the UK for mushrooms as a treatment for depression.
http://www.hopkinsmedicine.org/news...cinogen_may_create_lasting_personality_change
Depending on the individual I would guess a steady regime of mixed psychedelics at low doses regularly taken only so often so as you don't become overly tired or familiar with the altered state would be the best bet for maximum mental enhancement.
Perhaps 2CX's, LSD, Mescaline, Mushrooms cycled with two to three weeks breaks in between, weed could be used on off weeks if it has a productive effect on you personally (rather than couch lock at some doses).
It is very hard to measure increased creativity particularly if the individual has not tried certain arts or activities both before and after using the psychedelics. It could also be more difficult to quantify for individuals who work in offices or sales and don't have any hobbies that require creative input, though you would expect their sales or performance to increase if they can be more creative in their work or perhaps simply because they are more happy/at peace.
I've been meaning to read the below study around Ayahuasca as it is something I would like to try one day after learning a lot more about it but haven't yet found the time (it's on my long list of unread writings of interest). An excerpt is below, I'm unfamiliar with any of the tests but overall it sounds like positive changes.
"Controls were actively participating in non-ayahuasca religions. Users showed higher Reward Dependence and Self-Transcendence and lower Harm Avoidance and Self-Directedness.
They scored significantly lower on all psychopathology measures, showed better performance on the Stroop test, the Wisconsin Card Sorting Test and the Letter-Number Sequencing task from the WAIS-III, and better scores on the Frontal Systems Behavior Scale. Analysis of life attitudes showed higher scores on the Spiritual Orientation Inventory, the Purpose in Life Test and the Psychosocial Well-Being test. Despite the lower number of participants available at follow-up, overall differences with controls were maintained one year later. In conclusion, we found no evidence of psychological maladjustment, mental health deterioration or cognitive impairment in the ayahuasca-using group."
http://www.erowid.org/references/refs_view.php?ID=8606&S=dmt&SField=all
It would be good if in the future when hopefully psychedelics are more widely accepted and available that studies could be done on best combinations for particular fields of work and art, of course some sort of mental compatibility test to drugs would have to be developed, envisage the following being available if desired by the individual;
Engineers building more cost effective and innovative structures on a psychedelic diet of LSD and 2CB, playwrights on ketamine and weed whilst finishing their scripts or our politicians and rulers sent on mescaline retreats to inspire the best policies for all.
I essentially meant what you said, that's a more accurate way of putting it, but if something greatly inspires one, they have more to work with and thus more potential for creating.