Commands starting with P is what you seek.

Usage: P[?.+-*cdilnsS] [file]  Project management
| P [file]          open project (formerly Po)
| P.                show current loaded project (see prj.name)
| P+ [name]         save project (same as Ps, but doesnt checks for changes)
| P- [name]         delete project
| P*                printn project script as r2 commands
| P!([cmd])         open a shell or run command in the project directory
| Pc                close current project
| Pd [N]            diff Nth commit
| Pi [file]         show project information
| Pl                list all projects
| Pn -              edit current loaded project notes using cfg.editor
| Pn[j]             manage notes associated with the project
| Ps [file]         save project (see dir.projects)
| PS [file]         save script file
| PS* [name]        print the project script file (Like P*, but requires a project)
| Pz[ie] [zipfile]  import/export r2 project in zip form (.zrp extension)
| NOTE:             the 'e prj.name' evar can save/open/rename/list projects.
| NOTE:             see the other 'e??prj.' evars for more options.
| NOTE:             project are stored in dir.projects

Example of save/restore session:

$ r2 -
[0x00000000]> f myflag = 0xdeadbeef
[0x00000000]> Ps myproject
[master (root-commit) cbcd6c2] default message
 Author: dacav <dacav@localhost>
 1 file changed, 687 insertions(+)
 create mode 100644 rc.r2
[0x00000000]> q

Starting a new session:

$ r2 -
[0x00000000]> Pl
myproject
[0x00000000]> P myproject
[0x00000000]> f | grep myflag
0xdeadbeef 0 myflag

The project saving is done using git, as suggested by the Ps output. I think using git is a great idea, and I like how Pd can tell what happened in a project over time.

I figured a quick way to figure out where the saved project (repository) are: use P! and then pwd at the shell prompt. I guess there's some variable to tweak the defaults.

Also, the original question is quite old, and even trying Po will result in a deprecation warning these days.