There are a few approaches to culling the routing table. You can do it either statically or dynamically, according to your needs.
1. Filtering based on upstream communities
Slimming down the Internet routing table
2. Filtering based on region
BGP filter for North American routes
Substitute prefixes for applicable region(s). Each region is about 200k prefixes. For more granularity use a geolocation service to select prefixes and/or ASNs.
3. Using flow information to install only top routes
4. Aggregate the routing table
According to the weekly routing table report you can aggregate announcements to about half the number of prefixes. You need to roll your own software to preprocess the BGP feed. There are some tools out there, but I couldn't find a blog post about it with a quick search. If you have one, please share!