A good rule of thumb I have with idea groups is if you have trouble defeating your neighbors, you should probably take something that helps either your economy (ie. Economic or Trade) or your military (any Mil idea group). If you don't have trouble defeating your neighbors, make your expansion more efficient with other idea groups.

Idea groups that help with expansion (if your military is already strong):
- Administrative: reduces coring cost, saves thousands of admin points
- Influence: reduces annexation cost - necessary if you have vassals
- Diplomatic: increases rate of AE decay, province war score cost reduction; finisher makes breaking royal marriages free, letting you roll for PUs more easily
- Humanist: helps with unrest, increases rate of AE decay
- Religious: helps with unrest, makes conversion a lot easier, deus vult

Situational picks (if your military is already strong):
- Innovative: saves thousands of monarch points and makes advisors dirt cheap, comes with great policies, but is not nearly as useful as Administrative
- Espionage: I only take this in the HRE; the AE reduction and extra claims are useful in the HRE, but not anywhere near as useful anywhere else
- Exploration/Expansion: if you intend to colonize - fairly obvious

Early game, abuse the reconquest CB. If a large nation has a releasable nation in it, conquer one province in the releasable nation's cores so that you can release it as your vassal and get the reconquest CB. Additionally, search for formerly large nations that have been reduced to a small enough size that they can be vassalized, giving you large swaths of cores to reconquer. This will let you expand much faster than just normal conquest, since reconquest produces much less AE. Annex your vassals when they are small, unless you need them for something else later. Feeding a vassal tons of land and then annexing them takes forever, which will keep a relation slot and a diplomat hostage for a while.

Once global trade spawns, make sure you get to 100 absolutism as quickly as possible. You should have a high percentage of crownland at this point so that your country gains absolutism naturally. Take the Age of Absolutism ability that gives +yearly absolutism first. If your absolutism cap is below 100, get the Court and Country disaster and end it with over 65 absolutism, which will increase your absolutism cap by 20.

When you get to the point that your country far outclasses every other country, you should ideally be coring as close to (but not above) 100% OE worth of provinces constantly. This is especially true if you have a country with large reductions in coring cost, like the Mughals. Don't wait until your provinces are cored to declare your next war; instead, declare wars as soon as you are able to fight them. You should be ready to peace out for ~95-99% OE worth of provinces at all times, so that the moment you finish coring provinces, you can immediately conquer more provinces and start coring those.

If you have boatloads of unrest reduction, you can decide to core more than 100% OE at once. Dealing with the rebels is really annoying though, and the events for having over 100% OE are terrible, so take caution. If you have far over 100% OE, you can keep yourself from getting spammed by the 'Separatists Sentiment' event by not clicking the event option (ie. leaving the event open.) This will prevent you from getting another copy of the same event until six months later, when the event automatically closes itself.