middlewares.py

import os
import random
from scrapy.conf import settings
class RandomUserAgentMiddleware(object):
    def process_request(self, request, spider):
        ua  = random.choice(settings.get('USER_AGENT_LIST'))
        if ua:
            request.headers.setdefault('User-Agent', ua)

class ProxyMiddleware(object):
    def process_request(self, request, spider):
        request.meta['proxy'] = settings.get('HTTP_PROXY')

settings.py

# More comprehensive list can be found at 
# http://techpatterns.com/forums/about304.html
USER_AGENT_LIST = [
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.7 (KHTML, like Gecko) Chrome/16.0.912.36 Safari/535.7',
    'Mozilla/5.0 (Windows NT 6.2; Win64; x64; rv:16.0) Gecko/16.0 Firefox/16.0',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_3) AppleWebKit/534.55.3 (KHTML, like Gecko) Version/5.1.3 Safari/534.53.10'
]
DOWNLOADER_MIDDLEWARES = {
         'myproject.middlewares.RandomUserAgentMiddleware': 400,
         'myproject.middlewares.ProxyMiddleware': 410,
         'scrapy.contrib.downloadermiddleware.useragent.UserAgentMiddleware': None,
    # Disable compression middleware, so the actual HTML pages are cached
}