Keep the actions you want up top
Actions in Quicksilver come from different sources, including plugins, Services, custom actions and Quicksilver itself. New actions have a tendency to insert themselves just above the default action for a particular object type. Great for discoverability, detrimental to muscle memory and the ability to 'act without doing'.
Get back to working without effort.
Make an action the default
Find an object type in pane 1. Object types are not limited to text, URLs, files & folders, apps and email addresses. The default action in pane 2 will often be different for each object type.
Tab to pane 2 and, without typing, scroll down the list until the desired action is selected. Click the small grey dot next to its list entry and click 'Make Default'.
'Paste as Plain Text' might be the default action for text objects, and 'E-mail Item…' for email addresses, but what about pasting an email address? Grab some text in pane 1, tab to pane 2 and hit 'p'. Find 'Paste as Plain Text' in the list and click the grey dot once more. Click 'Set as Default for "P"'. Whenever an object is selected in pane 1, hitting ⌘⇧P will paste it plainly, no matter what its default action is.
Reorder actions in Preferences
The Actions panel is a scary looking beast, and is tackled in (the still somewhat scary) Action Actions blog post. For this post though, a quick setup guide.
Open Preferences and click Actions in the sidebar. By default, 'All Actions' is selected and the search field is focused. Type, or partly type, the name of favourite actions and make a note of their 'Rank' numbers. Clear the search field, find the actions, and drag them to the top of the list.
In Quicksilver, certain actions only appear for certain objects. It's better to have 'Open' ranked above 'Paste as Plain Text', as it will never appear for text objects. 'Paste as Plain Text' will appear for almost everything, and is usually undesirable as the default action for apps.
The more specialised the action, the less chance it has of interfering with the default actions for common objects. Here's a simple ranking of **common actions*** that will keep them as the default for their (object types) or particular kind of object:
Run (Files & Folders) – AppleScripts
E-mail Item… (Send Directly) (Email Addresses)
Search For… (Search URLs)
Paste as Plain Text (Any Type) – text
…creates these default actions
Try dragging actions from installed plugins to the top of the ranking list. Changes will take effect immediately in Quicksilver's interface, and are easy to reverse. File Compression, Displays, User Interface, and Keychain actions are among the many that are unlikely to affect common objects' default actions.
*The 'Play', 'Show Contact', 'E-mail Item…', and 'Search For…' actions require the iTunes, Contacts, E-mail Support and Web Search plugins respectively.