I am in the middle of drafting a utility patent. For illustrations, I
considered using Adobe Illustrator and Autodesk Autocad. Due to the
time constraints and higher learning curve of these software, I would
like to outsource the drawing part (US or abroad). However I fear to
run the risk of losing secrecy of the patent.

Any suggestion on how exactly to maintain secrecy/ownership while
outsourcing drawings?

Hi  tinypatient,

Any suggestion on how exactly to maintain secrecy/ownership while
outsourcing drawings?

Yes!  First let me say that it would be best to consult with your
attorney or patent lawyer to make sure everything is done correctly. 
Please see the disclaimer at the bottom of this page explaining that
GA does not offer legal advice but rather information.  I'll do the
best I can to lead you in the right direction but it will be up to you
to cover your bases.  Regarding your question, the general answer to
it is not difficult, but finding informational websites which aren't
there just to try and sell you something has been more of a challenge.
I've copied and pasted relevant sections below but please click on the
links for full details

Briefly, what you will do is draw up a Non-disclosure Agreement (NDA)
or Confidentiality Agreement before the consultant's work begins. 
This agreement will serve to insure that whatever you disclose to the
person or company will be kept secret between you and will not be
disclosed to anyone.

>> What is an NDA?

Understanding Confidentiality Agreements
" Confidentiality agreements, sometimes called secrecy or
nondisclosure agreements, are contracts entered into by two or more
parties in which some or all of the parties agree that certain types
of information that pass from one party to the other or that are
created by one of the parties will remain confidential. Such
agreements are often used when a company or individual has a secret
process or a new product that it wants another company to evaluate as
a precursor to a comprehensive licensing agreement. Or, perhaps one
party wants to evaluate another's existing commercial product for a
new and different application."
http://www.tms.org/pubs/journals/JOM/matters/matters-9405.html

The purpose of an NDA is to create a confidential relationship between parties.
http://inventors.about.com/od/nondisclosure/a/Nondisclosure.htm

Initial Idea Protection
Non-Disclosure (Confidentiality) Agreements
"Once you have shopped and decided which company you are going to work
with, naturally you must reveal the details of your idea and how it
works in order for the company (for example: a prototype maker) to do
his job. At this point, use the Non-Disclosure Agreement to protect
yourself before any work is started. In this way, you are using the
Non-Disclosure Agreement "selectively" throughout the period prior to
applying for a patent.
In recap, the Non-Disclosure Agreement should be used after you have
made the decision to work with a certain company or individual - who
absolutely needs to know how your product works in order to perform
their special craft. Working this way, you'll narrow down the amount
of people that know about your product, and will greatly reduce the
risk of someone stealing your idea."
http://www.inventionconvention.com/ncio/inventing101/003.3.html

Confidentiality Agreement
"A confidentiality or non-disclosure agreement is crucial for an
inventor or any other party who needs to protect confidential
information."
This page is divided into three parts:
    * important elements of a confidential disclosure agreement,
    * sample agreement containing these elements, and
    * alternative sources for information on confidentiality agreements.
http://www.bitlaw.com/forms/nda.html

Non Disclosure Agreements
"A Non-Disclosure Agreement (also known as a Confidentiality
Agreement) is used when someone with an unpatented idea shows it to
another party, and wants that party to maintain as confidential any
information. Find non disclosure forms and samples."
http://inventors.about.com/od/nondisclosure/

How To Use a Non-disclosure Agreement
http://inventors.about.com/cs/nondisclosure/ht/NonDisclosure.htm

0.24 How should one use non-disclosure agreements?
http://www.patentcafe.com/faq/index.asp?id=17&sid=17#0.24

>> Samples & Forms for Sale

Confidentiality Agreement
http://www.victorwhite.com/Capital/CapaitalR_NDA.htm

NOLO eform kit $16.99
http://www.nolo.com/product.cfm/ObjectID/FCE74CB8-AFF0-4A59-BD92CD66ECB54CBA/310/119/

Sample NDA
http://www.victorwhite.com/Capital/CapaitalR_NDA.htm

Confidentiality Forms and Legal Agreement Templates
http://www.ipbookstore.com/forms.asp

Confidentiality Agreement
http://www.lawdepot.com/contracts/confid/?pid=google-confid_ca-nondisclosure_b

Confidentiality Agreements
http://www.ilrg.com/forms/confidagree.html

I was glad to work on this for you, I hope you find it helpful. If you
have any questions, please post a clarification request and wait for
me to respond before closing/rating my answer.

Thank you,
hummer

Google Search Terms Used:  nda non-disclosure agreements
confidentiality forms patents templates

The answer is a lot easier than stated above.  There are many
companies that specialize in patent drawings.  They will keep your
invention confidential and do a good job at the same time.  Patent
drawings have special requirements and these companies will do them
properly from the beginning, without having to redo them after
objections from the USPTO.

Don't be paranoid.  Call a local patent law firm and ask who they use
for drawings.  Get a few suggestions and then ask to look at their
work.  If you use a reputable company there is no problem.

P.S. -- Getting professional drawings does NOT count as a public disclosure.

Good Luck