1. Increase memory limit (Rackspace provides 32 MB by default). I use the following at the top of my .htaccess files:
   # 
# Overridden PHP settings:
#
php_value memory_limit 128M
php_value max_execution_time 60
php_value post_max_size 128M
php_value upload_max_filesize 128M
    
2. Uncomment RewriteBase / in .htaccess to get Clean Urls to work
3. Turn on caching -  Rackspace's Cloud Sites use "computing cycles" for billing so make sure to turn on caching for live sites. This of course won't be an option for development sites but since there is no shell access and most developers use Drush these days, why develop sites on Rackspace?
4.  Ditch mail(). Rackspace has a reported issue with it, to get around download SMTP Auth module for sending reliable mail.

Also, if you are using Field Collection with D7, you might need to get a patch.