diff --git a/README.rst b/README.rst
index a085287c..1b81514b 100644
--- a/README.rst
+++ b/README.rst
@@ -938,9 +938,10 @@ Sessions
 ========
 
 By default, every request is completely independent of the previous ones.
-HTTPie also supports persistent sessions, where custom headers, authorization,
-and cookies (manually specified or sent by the server) persist between
-requests to the same host.
+HTTPie also supports persistent sessions, where custom headers (except for the
+ones starting with ``Content-`` or ``If-``), authorization, and cookies
+(manually specified or sent by the server) persist between requests
+to the same host.
 
 Create a new session named ``user1``:
 
diff --git a/httpie/client.py b/httpie/client.py
index 49fcdbd6..0f7daf79 100644
--- a/httpie/client.py
+++ b/httpie/client.py
@@ -29,7 +29,7 @@ def get_response(args, config_dir):
         response = sessions.get_response(
             config_dir=config_dir,
             name=args.session or args.session_read_only,
-            request_kwargs=requests_kwargs,
+            requests_kwargs=requests_kwargs,
             read_only=bool(args.session_read_only),
         )
 
diff --git a/httpie/sessions.py b/httpie/sessions.py
index 9824d233..e2410b4a 100644
--- a/httpie/sessions.py
+++ b/httpie/sessions.py
@@ -19,7 +19,13 @@ SESSIONS_DIR_NAME = 'sessions'
 DEFAULT_SESSIONS_DIR = os.path.join(DEFAULT_CONFIG_DIR, SESSIONS_DIR_NAME)
 
 
-def get_response(name, request_kwargs, config_dir, read_only=False):
+# Request headers starting with these prefixes won't be stored in sessions.
+# They are specific to each request.
+# http://en.wikipedia.org/wiki/List_of_HTTP_header_fields#Requests
+SESSION_IGNORED_HEADER_PREFIXES = ['Content-', 'If-']
+
+
+def get_response(name, requests_kwargs, config_dir, read_only=False):
     """Like `client.get_response`, but applies permanent
     aspects of the session to the request.
 
@@ -27,28 +33,31 @@ def get_response(name, request_kwargs, config_dir, read_only=False):
     sessions_dir = os.path.join(config_dir, SESSIONS_DIR_NAME)
     host = Host(
         root_dir=sessions_dir,
-        name=request_kwargs['headers'].get('Host', None)
-             or urlsplit(request_kwargs['url']).netloc.split('@')[-1]
+        name=requests_kwargs['headers'].get('Host', None)
+             or urlsplit(requests_kwargs['url']).netloc.split('@')[-1]
     )
     session = Session(host, name)
     session.load()
 
+    # Merge request and session headers to get final headers for this request.
+    request_headers = requests_kwargs.get('headers', {})
+    merged_headers = session.headers.copy()
+    merged_headers.update(request_headers)
+    requests_kwargs['headers'] = merged_headers
     # Update session headers with the request headers.
-    session['headers'].update(request_kwargs.get('headers', {}))
-    # Use the merged headers for the request
-    request_kwargs['headers'] = session['headers']
+    session.update_headers(request_headers)
 
-    auth = request_kwargs.get('auth', None)
+    auth = requests_kwargs.get('auth', None)
     if auth:
         session.auth = auth
     elif session.auth:
-        request_kwargs['auth'] = session.auth
+        requests_kwargs['auth'] = session.auth
 
     requests_session = requests.Session()
     requests_session.cookies = session.cookies
 
     try:
-        response = requests_session.request(**request_kwargs)
+        response = requests_session.request(**requests_kwargs)
     except Exception:
         raise
     else:
@@ -138,6 +147,29 @@ class Session(BaseConfigDict):
     def verbose_name(self):
         return '%s %s %s' % (self.host.name, self.name, self.path)
 
+    def update_headers(self, request_headers):
+        """
+        Update the session headers with the request ones while ignoring
+        certain name prefixes.
+
+        :type request_headers: dict
+
+        """
+        for name, value in request_headers.items():
+
+            if name == 'User-Agent' and value.startswith('HTTPie/'):
+                continue
+
+            for prefix in SESSION_IGNORED_HEADER_PREFIXES:
+                if name.lower().startswith(prefix.lower()):
+                    break
+            else:
+                self['headers'][name] = value
+
+    @property
+    def headers(self):
+        return self['headers']
+
     @property
     def cookies(self):
         jar = RequestsCookieJar()
@@ -149,6 +181,9 @@ class Session(BaseConfigDict):
 
     @cookies.setter
     def cookies(self, jar):
+        """
+        :type jar: CookieJar
+        """
         # http://docs.python.org/2/library/cookielib.html#cookie-objects
         stored_attrs = ['value', 'path', 'secure', 'expires']
         self['cookies'] = {}
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 5b4e154c..c3b33209 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,3 +1,3 @@
 tox
-httpbin
+git+git://github.com/kennethreitz/httpbin.git@7c96875e87a448f08fb1981e85eb79e77d592d98
 docutils
diff --git a/tests/tests.py b/tests/tests.py
index 79b85887..6b6e062d 100755
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -1370,6 +1370,26 @@ class SessionTest(BaseTestCase):
         self.assertEqual(r.json['headers']['Cookie'], 'hello=world')
         self.assertIn('Basic ', r.json['headers']['Authorization'])
 
+    def test_session_ignored_header_prefixes(self):
+        r = http(
+            '--session=test',
+            'GET',
+            httpbin('/get'),
+            'Content-Type: text/plain',
+            'If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT',
+            env=self.env
+        )
+        self.assertIn(OK, r)
+
+        r2 = http(
+            '--session=test',
+            'GET',
+            httpbin('/get')
+        )
+        self.assertIn(OK, r2)
+        self.assertNotIn('Content-Type', r2.json['headers'])
+        self.assertNotIn('If-Unmodified-Since', r2.json['headers'])
+
     def test_session_update(self):
         # Get a response to a request from the original session.
         r1 = http(