I've had great success with Mandevilla plants in my garden, and I think the key is to provide them with a warm and sunny spot, they love full sun to partial shade. The soil should be well-draining and rich in organic matter, I use a mix of compost and potting soil. Watering can be a bit tricky, you want to keep the soil consistently moist but not waterlogged, so I water mine regularly during the summer months and less often in the winter. Regular pruning and fertilization also helps to promote healthy growth and blooming, I prune mine back in the spring and fertilize every few weeks during the growing season.