While bunching up (see the answer by @Charlie Brumbaugh) works most times, it might be useful to have a method at hand that is less bulky for storing purposes.
You can make a Chain Sinnet, which is a slip knot into which you pull a loop and repeat.
When you fold your tarp or tent and find several lines together (making the bunches inconvenient as it makes too much of a bump) you can chain them together, by pulling all lines together into the loops and going on till the complete lengths are used. The final move of a chain that is in danger of getting undone is to pull the last end through the loop. But when putting the tarp in storage I would leave it without that last move to help in undoing when you use the tarp again.
For shortening while using the tarp, the Chain Shortening will work, but best when the line is used.
In that case you will need to fix the end of the chain, by pulling the end through or by tying an extra bit of string around the loop and the rest of the line.