There are multiple options here:
Bypass the switch
Use Tester101's instructions :)
Use the switch as an override-ON
In this case, turning the switch on overrides the timer and turns the light ON until the switch is turned back off, giving control back to the timer.
This can be accomplished by nutting all the blacks together and all the reds together in the timer box, along with all the whites and all the grounds. If you wish, you can also replace the switch with a single pole one -- join the incoming red with the outgoing black and a pigtail to one switch terminal and connect the incoming black to the other switch terminal.
Use the switch as an override-OFF
In this case, turning the switch ON allows the timer to control the light, while turning the switch OFF forces the light off. Connect the black wire from the incoming cable to the black wire on the timer, connect the red wire from the outgoing cable to the red wire on the timer, nut the black wire on the outgoing cable off, and connect all the whites together and the grounds together.
Get a 3-way timer instead of your existing one
In that case, the timer will have 5 connections instead of 4, or be able to operate without a neutral in the box. Keep in mind the latter timers have limitations on what loads they can control due to needing to send a "trickle" of operating power through the load in order to maintain memory.
Install a SPDT Relay to adapt your existing timer to the 3-way configuration
You'll need to use a UL listed relay like the Functional Devices RIBU1C for this job.
- The White/Yellow wire on the relay goes to the White wire on the timer and the White wires in the incoming and outgoing cables.
- The White/Blue wire on the relay can be nutted off as it's not used here.
- The White/Black wire on the relay goes to the Red wire on the timer.
- The Yellow wire on the relay goes to the Black wire on the timer and the incoming Black wire from the power source.
- The Blue and Orange wires on the relay go to the Red and Black wires on the outgoing cable.
- Finally, the Green wire on the timer goes to the bare ground wires on the incoming and outgoing cables.
(You also may need a mudring or extension box in order to fit all the wires inside it -- run the box fill computations please!)
Note that with this configuration, the outside switch will invert the sense of the timer's operation -- flipping the switch will convert timer ON times into timer OFF times and vice versa.