What are good Sandwich Sudoku strategies?

Scan the outside of the grid for the most constrained clues before touching anything else. A clue of 0 means the 1 and 9 sit in adjacent cells - Only eight adjacent pairs exist in a nine-cell line, and box constraints usually eliminate several immediately. A clue of 35 forces the 1 and 9 to the two end cells of the line. Very small clues are nearly as good: a clue of 2, 3, or 4 means exactly one cell sits between the 1 and 9 (no two distinct digits from 2-8 sum below 5), and that cell holds the clue digit itself.

For each remaining line, work out which positions of 1 and 9 are compatible with the clue. The gap between them determines the possible sums: one cell between them gives 2-8, two cells give 5-15, three give 9-21, four give 14-26, five give 20-30, six give 27-33. A clue of 25, for example, needs at least four cells between the 1 and 9 - So they cannot sit close together, which rules out many placements outright. Cross-reference rows against columns: a cell that must hold the 1 in some row arrangement may be banned from holding 1 by its column clue.

Once a line's 1 and 9 are placed (or reduced to one arrangement), you know the exact length of the sandwich. Now list the sets of distinct digits from 2-8 of that length that sum to the clue. Three cells summing to 9 must be {2,3,4}; two cells summing to 15 must be {7,8}. These restricted candidate sets behave exactly like Killer Sudoku cages and combine with naked pairs and box eliminations.

After the sandwich constraints have seeded candidates, the endgame is ordinary Sudoku: naked and hidden singles, pairs, and pointing eliminations. Re-check the clues whenever you place a digit inside or just outside a sandwich - The remaining sum keeps shrinking and often forces the next cell directly.