This depends on what level you are aiming to be or if you are making changes to your class. However, if you are staying at level 21 for now buffing your Death Saving Throw bonus extremely high and utilizing the Unfailing Vigor feat would be the simplest method, but can often rely heavily on your rolls.
Regain Hit Points: When an adventurer is dying and receives healing, he or she is considered to have 0 hit points, and then regains hit points from the healing effect. If the healing effect requires the adventurer to spend a healing surge but he or she has none left, the healing still helps a bit: The adventurer’s hit point total is restored to 1.
Unfailing Vigor: When you roll an 18 or higher on a death saving throw, you can spend a healing surge as if you had rolled a 20.
If you are looking long term, I would suggest looking into specific Epic Destinies. The Arcane Sword Epic Destiny gives you one option at Level 30.
Spellsword Perfection (30th level): Choose one swordmage encounter attack power that you know. You can now use that power as an at-will power rather than an encounter power.
If you take an Encounter such as Bravado Strike which lets you spend a healing surge whenever you hit, you can aim to hit with this move each round. This works even while you are out of healing surges. From there you can focus on items or feats that grant bonus hit points whenever you spend a healing surge, such as the Ring of the Risen
For an Elf Rogue Free Spirit, you only path is focusing on Death Saving throws and focusing specifically on a set of items and utilizing a great deal of money on potions. Here are the feats and items you can focus on.
Unfailing Vigor: When you roll an 18 or higher on a death saving throw, you can spend a healing surge as if you had rolled a 20.
Disciple of Death: +5 feat bonus to DST
Death Scorned: You gain a +1 bonus to death saving throws.
Periapt of Recovery - Gives +2 Item bonus to death saving throws. Any Level.
Potion of Spirit: Level 30: gain a +6 power bonus to DSTs. 125,000 gp
Verve Armor: Untyped +2 to DSTs. Any level armor.
Born of Shadow: +1 Untyped bonus to DST, only works in dim light/darkness.
Note: even with all these stacked bonuses you will need to avoid rolling a 1 on your Death Saving Throw and even then requires lots of funding for Potions of Spirit.