Basically, I go by "if it's on RCDB, it's a credit, otherwise it is not." In addition, I use the following modifications for more debated rides. To help with clarity, I define a credit as the actual mechanical system and a ride/attraction as the credit(s) plus any additional theming/landscaping elements specific to the attraction.
Multi-Tracked Coasters: If each track can operate independently, each one is a separate credit. However, if they share common elements, they are one attraction. For example, a ride like Colossus would be one attraction, but two credits. If a coaster is a mobius loop, it is one credit.
Relocated Coasters: It counts as a different ride, but not a new credit.
Modified Coasters: In order to qualify as a new credit, the layout needs to change significantly or most of the track needs to be replaced. Repainting/renaming/retheming a ride, changing/reversing the trains, adding/changing restraints, and modifying operation do not make a ride a new credit. Depending on what was done, it may qualify as a different ride as well. Examples: New Texas Giant, Phantom's Revenge, and Space Mountain would all count as new credits. Boulder Dash, Goofy's Sky School, Intimidator 305, Space Mountain: Mission 2, Superman: Escape from Krypton, and X2 would not.
Traveling Coasters: If I can verify that it is a unique ride, I count it. If I'm not sure, I don't. If an attraction is replaced by an identical model, it counts as a new credit once I can verify the ride is actually different.
Powered Coasters: As long as the ride is listed on RCDB and is gravity-influenced, I count these as credits. Coaster-like powered rides that are not listed on RCDB do not count.
Flume Rides: If the ride has a significant coaster portion where it is locked to rails (such as Journey to Atlantis), I count it as a credit. Rides that have coaster elements that are not listed on RCDB (such as Splash Mountain) do not count.
Gravity-Based Rides: Dark rides that are powered by gravity do not count unless they are listed on RCDB (such as Blazing Fury). Drop rides do not count.
Waterslides: Even though they are commonly called water coasters, Rockets/Master Blasters definitely do not count.
In all honesty, it doesn't matter how you count them, and I'm sure everyone counts differently. When someone asks me my count, I'll tell them something like "around 200" instead of a specific number. I don't care about how many rides I've been on, and won't go out of my way just to increase my count. I only keep track for personal interest.
Top 5 Steel: Bizarro (SFNE), Tatsu, Maverick, Kingda Ka, Top Thrill Dragster
Top 5 Wood: El Toro, Voyage, Boulder Dash, Thunderhead, Ravine Flyer II
Top 3 Parks: Busch Gardens Williamsburg, Holiday World, Cedar Point
TPR Trips: IntimidaTour 2010, North East Trip 2011, LeviaTHON 2012