Get a pad of big sized post-it notes and start doing some preliminary design before you even think about "we'll need common routines for these things", "this.pl" or "that.php" or any other technological concern.
Sit down with another developer and start hashing things out : if you'll need a login screen write "login" on a postit, and tack it to the wall. If you need a way for users to query test data, put "search for old tests" on an postit, and tack it to the wall. As you and your colleague free associate, you'll change the association of the notes ("The query page should come after the login, and maybe we should have a description of how the query page works, and some links describing some basic queries on that page...") -- this is why I like using postit notes, you can shuffle them around as needed and keep things neater than a paper sketch or a whiteboard.

After some time you'll have a good collection of postits that should give you a rough idea of how your project design should look. Take each postit and start fleshing out the design for that particular item, what ui elements need to be present, business logic, and so on, and you'll have a good start on a solid design.