Last season, my record in the 45-over National League (as it's called on ICC 2005) was:
Batting first: Played 7, Won 1, Lost 6*
Batting second: Played 9, Won 6, Tied 2, Lost 1
Batting first is harder because you don't know how many runs you need, whereas batting second you can pace your innings. Set yourself a target when batting first, a score which you believe you can defend, given the pitch, weather and quality of opposition. Then pace your innings and work towards that score.
If you are defending a small total, try restricting the opposition in the beginning, instead of going on all-out attack. Then, watch the required run-rate creep up, once it rises to about 5.5 to 6 an over required, the opposition will play more shots, so attack them at this point and you are likely to get wickets. Set yourself a target such as: 'give them at least 105 runs to chase off the last 20 overs', 'give them at least 60 to chase off the last 10 overs', etc.
When defending a big total, you could attack them from the beginning, because the chasing batsman will be aggressive right from the start. Also, bowl to their weaknesses.
*Didn't quite go to plan in my own matches though....