I'm going to move you to Game Design for this question -- while it does involve artists, this particular question seems to be more specifically focussed on the flow and game-play rather than just the graphics, which in professional development are often created by someone other than the designer who lays out levels.

I would suggest having a list of goals in mind when approaching level design:

• What sort of experience would you like the player to have? Do you want to encourage stealth? A direct approach? A mixture?
  
• Should there be one approach to the level, or should there be various possible approaches?
  
• (Mostly single player/campaign) Is there any particular narrative you need the player to experience? Certain things that must be discovered, or a particular encounter?
  

You can then start designing a level based on these goals. You might provide a single narrow path if you want the player to take a direct approach and have trouble, or you might provide large open areas with lots of cover if you want to encourage a stealthy approach. Put together a low-detail overview of the map laying out the basic details of what areas the map might feature.

You can then begin to refine the level and add details based on lower-level goals, approaching each section or area of the map. Things to consider include:

• Pacing: do you want the player to struggle with constant action, or will you provide points where the player can rest safely (whether that be absolute or just relative safety) between sections of action.
  
• Tension (strongly related to pacing): Can you force (or encourage) the player to take risks and get into more intense situations? Consider a shooter for example, where you might put the only health pick-up amongst a group of enemies to encourage engaging them.
  
• Potential tactics: can the player sneak around different paths? Does the AI have strengths or weaknesses that the player might consider exploiting -- and does the level provide the appropriate features to allow them to do so?
  
• Aesthetics and emotion: do you want to show the player something different, impressive, funny? Should they be allowed to take their time and absorb the scenery, or should they be forced to move on.
  

Once you've got the layout and details sorted out, you can either go about building the level or hand it over to an artist to add the aesthetic details: things like whether the cover you added should be rocks or crates. You might then consider running play-tests of the level and adjusting any problem areas -- players may discover unplanned paths or approaches, or expose the potential for adding new or varied experiences.



For specific techniques, I like to lay out levels on a white-board, adding and erasing details as I go. You can easily add or remove notes, and can make check-lists of the details above to check off as the level is laid out. If there is a narrative involved you might also consider a story-board of the main plot points, and ensure your level will take the player to them in the desired order.

You might also consider pen & paper prototypes, or building mock-ups in a level editor.

Hope that helps!

You might read through some of the stuff at worldofleveldesign.com for some ideas. It's very UDK-centric, but there is still a lot of good stuff there.